I think he assumes the factory production aspects of how fetal bovine serum is collected taints the karma of lab grown meat.

But that is indeed an entirely sentimental or spiritual concept. FBS doesn't contribute to the suffering of animals because it is a side product that is actually discarded most of the time. At worst, buying FBS means making the main production chain slightly more profitable.

In any case, I don't believe "clean meat" means less animal suffering than "cultured meat". "Clean meat" is just as unscaleable as cultured meat (currently) because of demands on labor and land. You still need to kill an animal long before its natural death, and you raise that animal in the first place to kill it. So I don't follow that particular argument.