He's addressing the problem with your hypothesis that "native games on the web will disrupt Steam" by pointing out that you're comparing apples (a technology platform) and oranges (a distribution channel). Steam doesn't exist because games don't use WASM.
I can't imagine I'd use Steam much after that. (And I guess facebook would do the same.)
As for payments, I already pay mostly through Google pay or through paypal. Steam isn't adding a huge amount of value.
Why you expect your Unreal-to-WASM product to succeed when Epic Games can simply build your entire product as a feature of the engine? You are building on their platform and not an open web stack, after all.
Despite having an open web, most people have elected to use platforms. At some point, people will defer revenue for simplicity.
https://www.theimmersiveweb.com/
This appears to be a 3rd party attempt to bring UE4/5 to the web.
I'd love to hear OP's take on how they think they can outpace or complement Epic on their own internal implementation. At face value I don't see how this isn't a f.lux vs. Night Shift type situation, where the official feature destroyed the market for third parties.