I can also attest that there are many 30k tanks starting to show up in the progression guilds on NA - The Shadowlands (East Coast). This can't be a fluke or everyone gearing wrong. There is something to what Epeshia is saying.
A lot of it comes from people referring to information on largely outdated or misinformed sites. I checked Noxxic for what they believed the best Shadow tank setup was and a lot of the advice there was at least 6 months out of date, considering what is known now about optimal gear setup, spec, and rotation. I've seen a lot of people use that as their baseline for performance and, sadly enough, they take it as canon rather than trying to determine what is optimal.
Just because someone uses and succeeds with a specific gear setup does not mean that it is optimal. Many of the tanks I see stacking loads of HP stack it completely arbitrarily. Most of them have no idea what the actual DR curves are, nor do they actually understand that DR exists on a curve rather than having an explicit soft cap chosen by some random person online without showing any math where you want to get to and then ignore from that point on.
Many of them that, ironically enough, come to me asking for advice, largely looking to be told they're doing it right. I've been told more than once by someone that thought that NPCs had crit chances and that TOR used a 1 roll system where sufficient shield and defense chance was needed to push crit off of the probability table that I have no idea what I'm talking about *after* they came to me asking what they needed to do to become a better tank. Many people do research at a single site, without actually bothering to understand the underlying mechanics of the game, and a lot of those sites that aren't these forums generally consist of a single person's math using often outdated/misinformed math or biased metrics (like TtK or eHP, neither of which is of particular use in TOR).
There will *always* be people using and succeeding with sub-optimal load outs and rotations because the gear doesn't *require* optimal gearing to succeed at the hardest content. It just requires "good enough", which, oftentimes, hp stacking *is* even if it's not optimal or even close to it.