Yep, it would be nice if they had used standard terminology. Example: a "virtual router" that you could configure (with internet, NAT, connections to other VPCs, firewall rules, etc.) Instead, you have all sorts of one offs: "internet gateways", "NAT gateways", "egress only internet gateways", "transit gateways." We're going to wind up with a whole generation of engineers that only understand "cloud" and not how things actually work.
My suspicion is that they have distinct names/skus because (a) they're billed differently (b) they represent different security choices. If I grant a developer's account the ability to create "virtual router" entities, that means said developer can do all kinds of things I may not approve of. If I grant them only the ability to create "Internet gateway" entities, I know exactly what I'm getting into and (more or less) what I should expect the bill to be for that
I do agree with you that having a separate "egress-only Internet gateway" for IPv6 is dumb and confusing. I'm sure there's some number of pizzas which explains this Conway's Law instance
