Well there you go — I am in favour of fake fur, fake leather, pretend chicken and I also completely support lab grown meat.
As for your other argument, I put forward that it makes taking animals into consideration more attractive as it appears less black or white and less impossible to achieve. It is reassuring for omnivores to think that they may still be able to enjoy eating bivalves every now and then. It also helps present anti-speciesism as more rational and less dogmatic.
I have never promoted eating sentient seafood and I don’t intend to but I have helped a significant number of people go reducetarian, vegetarian and vegan. I am happy when they don’t got vegan overnight as those who go at their own pace and think it through are more likely to remain veg*n.