Your mouse and keyboard excuse is an awful one. Look at the incredibly competitive game, Fortnite. People are playing on their smartphones against people using keyboard/mouse and controller and no one has been complaining.
I just think it's a massive fallacy to say that when a current game proves you wrong.
I see what you're trying to do here. Using my own words and arguments against me. It's cute I'll give you that! But unfortunately it won't work.
In Fortnite crossplay is an opt in. When a mobile user is playing against PC users it is because that mobile user is partied up with a PC player. That mobile player is choosing to play against unfair competition and he probably doesn't care because he just wants to play with his friends. If you're just jumping into a random game of fortnite on mobile you are playing against other mobile users because anything else would be completely unfair.
And when that mobile player parties up with his PC friend it is the mobile user who is getting matched up against other PC players, not the other way around. Do you know why? Because even Epic knows that it's inherently unfair for mobile players to go up against PC players, so if they're going to mix and match they're going to make sure that the burden is on the mobile player who chose to put himself in an objectively worse position competitively speaking.
For fallout 76 I believe Todd Howards said at E3 that he doesn't want players to see a server browser. He wants populating the world to be as natural as possible. So that probably means that an opt in system doesn't work with the team's vision. They'd probably be ok with naturally populating worlds with Xbox and Playstation users mixed together because they're all just using analog sticks, while he would be less inclined to throw PC users into the mix because the competitive advantage a mouse and keyboard has over a controller is insanely high. You don't have to agree with the decision, but it certainly doesn't mean that choosing not to enable crossplay between PC and console means it would never have happened between Xbox and Playstation. I mean Todd Howard literally said crossplay is not a thing because of Sony's stance. I'd be inclined to believe the devs on their own game.
So no. The m+kb vs controller argument is valid. Even Fortnite takes it into consideration with its matchmaking. It was a nice, cute try on your part though. Better luck next time!