I have vimium installed so ? displays viumium help. This is a remainder that still there is a gap between native apps and in-browser apps. Yes, it is shrinking, but browser was never designed as the platform the run full-featured applications. So expect the unexpected.
But, true, comparing to other software of this kind this one looks really good.
I think they are referring to the actual button in the UI, which is in the button right, not the "?" key on your keyboard which might or might not be in the bottom right in your keyboard.
Anyway, as a user of Vimuim, you're surely used to keyboard shortcuts already being funky, and you know about the "escape" mode it surely offers in order to trigger keyboard shortcuts that conflicts with your own bindings. Just use that instead of complaining as it's your own setup that makes things difficult, not the website owners fault really.
It's a bit like writing something like "Hey, I use a extension that makes all elements background black on page load, and on your website, since the text is already black, makes it very hard to read stuff"
I say this as a vim-keybindings-in-browser user myself too.
Does that mean they shouldn't ever change to become a platform for full-feature applications? Is there a reason why browsers shouldn't evolve and grow and add new capabilities? We don't believe that operating systems should be static once they've been released, or that applications should never add new features that enable them to run scripts or plugins. What's wrong with browsers doing more than Tim Berners-Lee envisioned 30 years ago?
Sure, browsers were once designed to view docs. Now they do more. That's a good thing even if it throws up some design, UX, and security challenges.