You are correct that Gyarados doesn’t get a reliable Flying-type STAB, and that being a Flying-type doesn’t necessarily mean we get a STAB move. However, to maximize its ability Tinted Lens, I would argue that CAP30 needs to have access to a strong Flying-type STAB to be best product. The insane strength of Tinted Lens Flying-type moves is a huge draw, and Tinted Lens Poison-type moves just doesn’t come near that. This is especially true with our secondary typing, Poison, as it doesn’t offer nearly as much with Tinted Lens due to Steel types being immune to it. Without Tinted Lens Flying-type moves, I’m not sure what strengths are left that CAP30i can actually use effectively.
You're both missing an important point, which I bolded below:Reminder that 30b will be sharing it's movepool with 30i. What you're suggesting here is, in fact, putting restrictions on the movepool of 30i, to try and preserve some of the few answers that corrosion could have. I feel like it should go without saying that this is far from ideal.
For example, if 30i is a physical attacker while 30b's attack stat is terrible, we can effectively give 30b no Flying STAB without restricting 30i by giving them both Gunk Shot, Brave Bird, and Sludge Bomb but not Hurricane.It's worth noting that being Poison/Flying type does not necessarily mean you get either Poison or Flying STAB. infamously has Bounce as its best Flying STAB, and itself has Cross Poison as its best Poison STAB. So if we as felt it was really important to us that we're walled by or or whatever, we could simply not give 30b both STABs. Physical/Special split means this need not interfere with 30i.