Sure, but it wasn't a "mistake". Stroustrup absolutely wanted a C with classes, and that meant tight integration with C toolchains. Symbol mangling was the clever idea invented to implement that very deliberate choice, not a fortuitous happenstance.

I'm pretty sure @pjmlp is using the word "mistake" to say "a decision that turned out to be bad". English is not my native language, but judging by the ways I've seen it used and what dictionary definitions I can find, it seems quite acceptable.

Stroustrup made the decision on purpose and consciously, but it turned out to have disastrous effects.

Yep, you got it right.

Actually name mangling predates C++.