Also, we have multiple subs. Revealing one temporarily does not compromise our deterrence
I think what the person you're replying to meant was something like:
"These subs are known to exist by Russia, and their particular sound signatures may NOT be known to Russia. So, having one appear in a known location, Russia may now be able to link sounds previously unknown to them to this type of sound. With this new information, Russia may now be able to locate these types of subs, when that was previously not possible."
I'm not sure if they're right about this or not, so the issue may be moot if Russia already had this information. I do, based on my military experience, generally tend to agree about the OPSEC component of their statement though.