No, I would not have a problem with someone eating them and still calling themselves vegan but it seems that things like this don't cut it for being defined as (a strict) vegan. Veganism is based on not using any animal product rather than whether or not some animal products can be considered immoral.
One issue could be that the catching of mussles may lead to other things getting killed in the process. But if that's not true, it's probably a (relatively) ethical food to eat.
The other question is how sure you can be that mussels aren't sentient or whether they have some spiritual value. I actually went on a beach holiday where all the restaurants were mostly fish and seafood one month after converting to a vegan diet and I was still transitioning. I decided not to eat any of it because of this question of uncertainty about what is sentient and what isn't and how you know for sure, and how it's better to give the benefit of the doubt.
I haven't eaten any seafood since the first month of being mostly vegan. I have never really thought about this a lot since then.
I do think some types of seafood are at the very least much less immoral than meat and fish, but not sure it's completely 100% clean conscience either.