Because he got a very common symptom of steroid/PED use appear out of nowhere. You CAN get gyno without PEDs, but it is usually in puberty.

Also if it was non-PED caused outside of puberty it would be very minimal and would gradually appear. Adesanya's appeared quite suddenly and was VERY big and noticeable. Exactly what you would expect if he was introduced to a new PED to take that he'd never done before.

This is somewhat subjective but I believe he put on a little more muscle as well when this was first noticed, compared to his physique before. I thought he was skinnier before that appeared. But even if he wasn't, a lot of PEDs can increase strength, power, speed, recovery, stamina, etc, with very minimal changes to your muscle size.

It also happened first before his fight with Paulo Costa (or Romero can't remember 100%), which would make sense seen as he was fighting someone with a huge amount of strength and power, and is likely on PEDs also.