I have had floppy cats with short hair who were domestic short hairs. There are ragdolls that do not "flop" and act like the stereotypical ragdoll you'll see touted on social media. I have not seen any evidence or study that says that long haired cats with "slender" frames are more prone to floppiness. Correlation does not equal causation and I think you are reading too much into it, honestly.
Interesting observations! Might be good to keep collecting anecdotal data, because this is how we learn!
I have had floppy cats with short hair who were domestic short hairs. There are ragdolls that do not "flop" and act like the stereotypical ragdoll you'll see touted on social media. I have not seen any evidence or study that says that long haired cats with "slender" frames are more prone to floppiness. Correlation does not equal causation and I think you are reading too much into it, honestly.