"Only". Heh. Bit more than just some minor implementation detail, seeing has until Ruffle there's never been a successful 2nd implementation. ;)
Plenty of false starts and half arse attempts to make one, but none that gained traction until now. :/
If Adobe had committed to turning Flash into an open standard back when it was still relevant (certainly before 2010, maybe as late as 2013), various companies like Google, Microsoft, Apple and Mozilla would have committed dozens of full-time engineers towards working with Adobe to document flash as a proper standard. And then those engineers would have gone back to their browsers to create implementations.
None of those companies would have made a 2nd implementation of flash back then without Adobe's permission. Not only did they all have working relationships with Adobe (mostly around keeping adobe flash player updated and secure), but there was a risk Adobe might sue them.
Also, without an offical standard, Flash was a moving target. Part of the reason why Ruffle is viable now is that Flash is dead, and there is no offical implementation that might change at a moments notice.
I remember a number of people pushing Adobe to transform the runtime into an open standard. Then every browser would have been allowed to create their own implementations that actually fit with their security model.