> limitation to all sender-obfuscating cryptocurrencies

Only those that allow sending coins to recipients without their explicit approval. On pure Mimblewimble blockchains, the recipient must sign for receipt and is much less likely to accept poisoned funds.

It's irrelevant

Sorry, I didn't watch the video, but couldn't that be avoided by churning a few times?

Sort of, but not really. It is a very nuanced probabalistic attack. The problem is fundamentally that you need others to use your outputs as decoys in order to have plausible deniability.