FUD implies that a decision was mostly based on people's irrationality or misinformation about what the patents clause does or doesn't do. It's a lazy assumption not made by someone who has followed the discussions in the WP community, so yes it was both ignorant and arrogant.

> have explained that you are strictly better off with the PATENT grant

This point is irrelevant even granting the assumption that it does. You're inspecting the wording of the patent grant and gauging whether it is theoretically effective in deterring patent litigation and thereby conferring some kind of benefit to users of the software is (correct?). Well over 99.9% sites that use WordPress never have to worry about relevant patent litigation to start with, it's completely and utterly beside the point. Somehow this has blossomed into an argument that basically says 'our OSS software needs to come included with patent clauses for better protection'. It's ridiculous.

Why the FB license signals a bad fit comes from asking different questions.

Why is it there in the first place?

Because FB is a megacorp and operates like one, it is there to protect the interests of a megacorp. Not a slight on FB, but also not something we should welcome to package with every copy of our pure OSS software.

What is a consequence of non-standard clauses? Businesses and teams feel compelled to take up legal counsel. They may find that it turns out to be totally fine, as even Automattic found when it adopted React for Calypso. But others don't and the mere fact that the adoption of software is necessitating legal counsel adds an unnecessary and negative cost to choosing WordPress as a platform.

What kind of precedent does it set?

That non-standard additions to OSS licenses are fine for companies to impose. Alternatively, FB is a special case.

Does it undermine WP approach to OSS licenses and promoting these?

In WP we take no exceptions to GPL compliance and compatibility, no exceptions for obvious reasons. No business enjoys special privileges or exceptions. That's by design, WP stated goal is to democratize the web. Unstated goals mean that everybody is on equal footing, a level playing ground. You want to use WordPress? You can do so unreservedly as long as you convey the same exact freedoms that come with a GPL compatible license. No exceptions. Whether the patents clause is benign or not - it is a big fat sign that says "FB enjoys special privileges here". It complicates the picture by offering up a contradicting component by way of a non-standard clause. It introduces friction on a point where there should be no friction at all. It also encourages an ecosystem where multiple clauses are introduced with companies adding their own wordings to protect their business, it doesn't scale. No good reasons to support something that doesn't scale.

Who put the clause there?

A company that is hostile the open web, hostile to democracy, hostile to users and with a history of undermining startups while enjoying a monopoly position. It is entirely in-congruent with a world that WP wants to be in. It causes many people that use WP for whatever reason to be suspicious and wary of affiliating with or critically depending on anything this company does. That's not FUD, it's a reasonable position based on a cold hard assessment of what FB is.

But doesn't company X choosing React and determining it to be benign mean other users are worrying for nothing?

Any particular business might be fine with React, it all depends on what their project is and what their goals are. Other companies will explicitly avoid React for good reasons, some have done so on the basis of the patents clause. WP adopting core is making a decision for all people using WordPress and all future projects that would or would normally have chosen WordPress. It's a monumental decision, inviting different considerations. A platform that has pinned its success on offering something that is both free and licensed in a way to convey freedoms while working toward leveling the playing field can't just adopt a special license for all of its users merely on the basis that the software attached to the license currently leads the pack.

Everybody agrees software patents are bad, so why not embrace the patent clause?

I would guess virtually a 100% of WP users are anti-software patent. But clauses that protect corporations are not the solution.