Not sure if this is known yet, but im just going to leave this here.

I was able to replicate it 2 other times before this, not sure why it happens, but the sliding into the boss when he slams his head down seems to do the trick

I just got this is well, but it was when they attacked by slamming their heads together. I was under the right head then, but you got it from the left side so I guess it doesn't matter.

I believe I got it when the attack put me at critical health (when you hear that distinct sound effect) and it looks like you got it then as well. Just speculating, but maybe your hitbox gets certain properties when that happens, so collisions get wacky?

I had gotten it a couple of times before, and once without being near critical, I think it has something to do with a slide kick on the same frame you take damage from above, the game tries you boost you down, and the kick tries to force you backwards, causing a clip, thats my guess.

So, another streamer got the same thing but in a different way, he never did a slide, which leads me to believe @Kirthar " I got it when the attack put me at critical health" might be true.

I did this just by falling through one of the gaps in the floor. Then i just walked back up until I was inside the right dragon that wraps around the tower.

I saw a clip of someone that got on top of the lower dragon with QCF kicks (what are people calling that tech? float kicks?) and then proceeded to just obliterate them in about 10 seconds with chained divekicks. They used Bunnymorphosis, if it's any use - I don't know if the time spent killing a Lili or two to try and get the shard would result in any time saved over trying to get the floor clip working. It might be possible to instead just switch to Lethal Boots (which you can get for free and relatively quickly from a chest near the first Zangetsu fight) after you have enough height to get on top of the dragon.

Tried to find the clip but the prick that posted it didn't name it something logical, it had some kind of snarky name so I'm not having any luck finding it with Reddit's useless search feature.

Hey there, while discussion here is fine, you're all encouraged to join the discord server: https://discord.gg/TPgNHPF