"There's two ways I can see you interpreting this.

"1) You are being extremely narrow in your focus and saying that you are talking specifically about viruses being transmitted as aerosols (although the overarching topic here is about whether or not masks are effective). And thus, since there was no research specifically about this, then there's no reason to recommend masks. This doesn't address the point that there's research that shows masks are effective against similar kinds of viruses.

"2) You actually are talking about effectiveness of masks and saying that viruses could not be transmitted via aerosol was to say that because they are not shown to be transmitted via aerosol, then there's no evidence that masks would work, in which case, again, we're back to the issue that there was research prior to 2020 that showed masks are effective against similar kinds of viruses."

Backing up a bit...

As far as I've seen, there are three primary routes for infection for respiratory diseases: 1) contaminated surfaces, 2) (large) droplets produced mostly by coughing or sneezing, and 3) (small) aerosol particles produced by normal activities like breathing and speaking.

The normal measures against 1) are avoiding touching possibly contaminated surfaces, washing your hands, and not touching your eyes, mouth, etc. And roughly speaking, that's about all you can do.

The normal measures against 2) are staying distant (i.e. 6ft) and keeping interactions short because the droplets do not remain airborne long, and covering your face when you cough or sneeze. Masks would certainly be helpful in the case of 2), but not especially so because a) the normal measures work fairly well, b) most people do not want to wear a mask[1], c) many people who do wear a mask do not do so correctly, and d) the supplies of medical grade masks were (are?) sketchy. (Both of the links provided are specifically aimed at b) and c), no?)[2] A study of 1000 students at an Australian university is interesting, but the advantage of a) don't necessarily overwhelm b), c), and d).

There are no normal measures against 3). The only useful measures are to avoid all contact with potential carriers, significantly improve indoor ventilation and air filtration (aerosols remain airborne for a very long time), and properly using medically-effective masks when interactions are required. Transmission by asymptomatic carriers is primary, hard epidemiological evidence of 3).

According to (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7322154/), asymptomatic carriers were reported at the end of January and significant confirmation appeared about March (https://wwwnc.cdc.gov/eid/article/26/7/20-1595_article). Shortly after that (April 3), the CDC recommended masks (https://www.bmj.com/content/369/bmj.m1422).

From your limited choices, my closest meaning is your 1). But,....

Now, put yourself in the place of someone making an official policy recommendation in, say, February or early March. You don't have hard evidence that masks are required, but you do understand that they will provide some marginal benefit. On the other hand, ensuring that masks are worn consistently and correctly is an uphill struggle (as we have seen over the last year). Further, the supply of medical grade masks where their use is required, hospitals for example, is not infinite. Oh, and you want to make the minimally invasive recommendation you can, because you actually aren't out to cause as much damage as possible by, say, killing the economy. Beyond that, you know that at some point you are going to be facing pandemic fatigue, where people stop taking the situation seriously and then things get very bad (as in last summer, last fall, and earlier this winter). What do you do?

As it turns out, they were wrong about some of their assumptions. Being wrong happens. It is not proof of an evil conspiracy or even of a conspiracy of stupidity. It's people who are pretty good at what they do, making what they think are the best choices, and being wrong.

Suggested Reading

https://www.bmj.com/content/369/bmj.m1422

https://news.ycombinator.com/item?id=25616014 (No, really, "The Plague Year" in the New Yorker is probably the best history of the pandemic so far, and explicitly touches on a lot of these issues---including medical professionals saying, "yeah, we were wrong.")

[1] I am talking about the US specifically, not Taiwan, Japan, or anywhere where mask wearing is more common socially.

[2] Everyone wearing pressurized, highly filtered contamination suits at all times would prevent essentially all cases of respiratory disease transmission. But no one is going to push that idea without a really, really good reason.

PS: While writing this, I ran across https://www.sciencedirect.com/science/article/pii/S019665532..., which is the first solid, halfway data-backed estimate of the prevalence of asymptomatic carriers: 1.82%