Well, it was a very close fight against an opponent who was 20-1 on a 14 wins streak, all while Pereira fought with Norovirus and a hand fracture, which he confirmed later (tho he said he wouldn't like that to be brought up because it was Ankalaev's credit)... I'm sure that fighting 4x for world title at 37 yo, when vs Khalil he was on antibiotics for 3 weeks of his camp and vs Ankalaev, he had a stomach virus that nearly forced him to pull off... And still gave Ankalaev his toughest fight, then yes, he's a badass. A fresh Pereira like vs Jiri Prochazka 2? Yeah for sure I'd bet on him vs Tom, not even a thought. Injury free, no weight cut, 243 lbs while being fit and healthy, having faced only elite opposition and champions and fighting near evenly with one of the best current LHWs isn't something HWs would be prepared to deal with... There's a reason Tom is avoiding when ppl ask him about Pereira wanting to move to HW despite that fight being the best he could get — because he is not confident he can beat Alex Pereira when Alex is healthy, specially seeing as Alex TDD has vastly improved...
Compare that with Tom's resume and you get nothing short of Alex Pereira... Less experienced, worse grappler than Ankalaev, his striking is more predictable as he goes into straight lines in rushes which plays right into Alex's counters... Specially with Alex weighing 110kg and healthy, yes, I think Ankalaev was and is the toughest test for Alex outside Jones... And if it's a healthy Pereira, you will see him as a HW champion.
110kg (243.8 lbs), healthy, and very fit... With TDD being at the top of LHW. Yeah there's a reason Tom said maybe he'd fight Pereira. Tom said as well he plans to retire sooner, that he won't stick for too long unlike some ppl due to cognitive issues, as per Tom. He said he plans on doing 4 fights.