Who’s punk what’s the score?”
Get outta here with the gatekeeping
1) Ethos
2) Aesthetics/look & feel
3) Musical sound
So someone could be punk as fuck ethos-wise but love listening to Yanni. Meanwhile a band could have a straight up anarcho-punk sound, but otherwise be white collar wage slaves
But this is the root of so much disagreement. When Green Day started ascending in the 80s, people were pointing out the poppy music as not being "punk". But that's just one variable.
Punk is about rebellion, pop-punk is a good umbrella term as they had rebellious vibes but to call Green Day punk is a bit of a stretch.
The irony extends to the fact that while someone can play in the style of pop without being mainstream, they cannot literally be pop until they're popular. But if pop requires being popular and punk requires shunning the mainstream, pop punk couldn't exist. The fact that it does is therefore a bit of a paradox.