Models before the announced date don’t have watermarking, so it’s unlikely. Now, if what you are interpreting is precursor work to develop the watermarking system, maybe?

I suspect it less insidious: Claude has/had the public sentiment of being the “better writer” of the models. At some point that distinction would have been diluted as other labs’ offerings “caught up” stylistically, unless Anthropic continued to tune their output…

I personally think they’ve pushed so far that they’ve overfit and lost the sweet spot they previously occupied.

That has no effect. Look up the math. Claude/CC are just bad.