So what? Even though engineers realize that, as long as you're talking abstract points, and not holding me to a deadline, I have no reason to modify my estimate. And even if I -do- modify my estimate, as long as I continue with that new estimation mechanism for some period of time, velocity will change accordingly and the PO can -still- make accurate determinations of when something will be delivered. If I consistently under or over estimate things, I am predictable. The more constistent I am, the less time it takes to be uniform, and thus predictable. By not pinning it to time, the PO can determine the average velocity, the current estimations, and have a largely accurate idea of when things will be done. The only time this ever breaks is when I have a reason to change my estimation strategy, which only happens when you start applying incentives for me to change it. Every time my incentives change, my estimates will change; don't change my incentives. Don't give me a date or a deadline.

We could do this with hours except hours actually translate directly to time. From them I can determine when you think I should be done and thus the implied goal; points I can't. Because of that, I have incentive to overestimate when it comes to hours, to give myself extra time 'just in case'. With points I don't; they don't imply an end time. Even if the PO's estimates are wrong, they can't get onto me, because I never had a due date. Because of that, my estimates tend to be consistent (even if inaccurate), and consistency = predictability. That's the goal, making it predictable when things will be finished (within a given tolerance; hours never give us that). Whether points are big, or small, the PO can determine "This team has an average velocity of 20 points, whatever size those points may be. That means I can expect around 20 points per sprint in terms of stories. I have 50 points I need to get done for this next release, that means I can expect it in three more sprints". All done without the engineers ever having a goal they are trying to make, no incentive to change their estimates, and literally the best predictability (not accuracy; you don't need accuracy) you can achieve, which, it turns out, is actually pretty damn good in practice.

I think you're missing the fact that points only matter relative to each other. There is no absolute meaning. The author hints at that but I don't think is explicit about it, assuming you already know that. As such, a team tends to be consistent with them, even if not objectively accurate. And their mistakes average out into something predictable, even if not accurate. Because there is no incentive, conscious or unconscious, to massage it.

Some people -do- decide to use t-shirt sizes for sizing, rather than points. It still works. This is generally equivalent to 3, 5, and 8 using points (fibonacci). Most people who use points say anything less than a 3 is wrong (make it a three), and anything larger than an 8 should be broken up because otherwise it's difficult to estimate (with -maybe- a 13). So use whichever you like. S, M, L, and maybe XL, if you want to map to four options, as you have with points. Though you need a way to aggregate them to determine a velocity; how many S = 1M, etc. That's why people tend to use points.

Basically, the author said, flat out, it's a psychology game. AND IT IS. What a PO needs from a dev is consistency in how they estimate, not accuracy. From that you can measure the actual work completed over time, and get a measure of velocity, which can be used to accurately predict the delivery of future stories, within a pretty good tolerance. Ensure the psychology for that consistency is there. Points are part of it. No goals, milestones, deadlines, etc, are part of it.