Costco is set up this way: their optometrists are contractors who get a flat fee per eye exam, and they don't know or care about what else you buy while you're there.
They might have a revenue sharing agreement, maybe, but they don't even encourage you to use it.
And beyond that, following the logic from the recent surgery thread: they see their patients see better. They see all the good cases, where someone walks out more confident and with better sight. Their product helps people. But then so do homeopathic placebos (to a certain (measurable) extent), and that's the hard part to figure out.
Of course, in this case everyone truly does see better when they walk out and what GP is wondering about are the long-term effects. This stuff is complicated, though I frankly have a hard time believing this claim of "just stop wearing glasses and you'll see better". Surely someone would have noticed that? But without doing a deep dive into the research here, it's all just speculation on both their part and mine.