The wording from the wikia is:
“Stone Resilience ➜ While raging, you gain temporary hit points equal to twice your proficiency bonus at the end of your turn.”
It is, I've played with it. You also get it when you enter the rage, but it doesn't stack, so it makes no difference.
Assuming that is accurate, that‘s going to be 8 HP at the end of your turn at level 9. You only gain those at the end of your turn in the round, not every turn of the round. Is it nice regenerating temporary HP? Yes, yes it is. Is it a crazy amount of temporary HP? No, not really. Is it such a large amount of temporary HP that it needs to be considered too strong and makes the Path of Stone Barbarian a mandatory party pick? Absolutely not. And if the level cap were 20, it’d be 12 temporary HP, so even tabletop in a max level campaign it’s a nice-to-have but not overly strong ability.
Given the updated difficulties of the endgame areas, 8 HP per turn on a Barb felt very insignificant. Shikkath deal crazy necrotic damage, bypassing the Barbarian's usual tanking ability.
As Tom Reneth pointed out, it’s the level 6 and 10 abilities that are the best, and even those aren’t the greatest things ever. Having your character’s own saving throws based on CON is nice, but a Paladin of the same level will be adding their CHA modifier to the entire parties saving throws. The level 10 ability is a welcome bump to AC, but maximizing it requires being surrounded, which often isn‘t great tactically. They are all nice personal survivability abilities, but even altogether they aren’t a “OMG, every group must have this character” kind of thing. It’s a solid tanking option (to the extent that tanking is useful) but “comically overpowered“ is an exaggerated claim.
The level 10 ability is really nice for the final area where you can't avoid being zerg ruched. It should give +3-4 AC consistently there.
I don't know if it is a bug or not, but Path of Stone Barbarians used their full Constitution save value for all saves, not Constitution in place of the ability score modifier for saves like it sounds like was intended. This effectively gave them proficiency with every single save and let them max out Con to get +9 to all of them. Not sure if it is still the case with the latest patch, but the wording of the ability suggests it is unintentiona.
Of course, probably bugged ability aside, Path of Stone wouldn't hold a candle to Totem Warrior on the tanking front. So it is hardly anywhere near 'overpowered' for Barbarians. Or even in Solasta, considering you can make any martial borderline immortal with the right support and have been able to since release.