Very good point, users do not care that a padding is rounded up or down when they switch from their laptop to their desktop, as long as the application is usable, understandable and visually competent.

But mindspace is much more important for interaction, so if their laptop is a Mac, their brain will be in "Mac mode", and "Linux" or "Windows" mode when on their other device. Respecting the platform's conventions will allow them to keep their cognitive load due to "fiddling" to a minimum.

I don't think the GP was talking about the app looking consistent with the rest of the platform, but the exact opposite: the app looking the same whatever the platform. Using Flutter allows them to have the app look the same on the browser as on mobile.

This means that at best it will look "Mac mode" for everyone, including Windows users; at worst it will look foreign to everyone.

Yes, I understand and agree.

My second paragraph was maybe a bit unclear, I meant that the user would expect platform conventions to be respected over application conventions. (especially if we consider platforms with different primary interactions)

Having a program look the same on a big screen with mouse+keyboard input as well as on a small touchscreen is a recipe for having a bad user experience on both.

I might be wrong, but I'm pretty sure Google Docs is already using a completely custom implementation of word wrapping, text layout, text selection, cursor placement, etc. It's not like it's just a <textarea /> with some CSS styles. Likewise, they seem to already have completely separate DOM elements that are invisible to normal browsers but can be read by screen readers. Based on the DOM on the new read-only preview document they link to, it looks like they will continue to use traditional DOM elements for some of the editing UI (just not the actual WYSIWYG editing area) and for screen readers.