. Having the completion style
remove all prior fontification sounds a bit extreme for an interface.
For me, it makes more sense to be additive, but I don't know what
Emacs' vision is on this topic.
It's up to the style. If you like it additive keep it additive, else do what Eli suggests.
`completion--twq-all` is doing fontification. This function doesn't
look like it's part of the completion frontend nor of the completion
style. Why is it doing fontification? Should it be doing
fontification?
It's part of the table. It does what it has done probably for many years. If you want to change that behavior for everyone, go ahead, but IME you'll eventually bother lots of people using simpler setups. If your style is bothered by it, remove it there just like Eli suggests. In lazy and in non-lazy situations. IOW the lazy optimization has nothing to do with this.
And if for some reason you're worrying about performance, don't. The lazy optimization is largely about avoiding premature upfront string allocations, not propertization. And it more than likely doesn't matter for file tables anyway which are small compared to symbol obarrays, for instance.