Well, this is just like the CIA or whatever attending Defcon. Google undoubtedly has some ulterior motive, whether it's to poach the best and brightest or to dilute the messaging, etc.
As a fellow Vivaldi user, you know what'll really make you sad?
There was a plugin that offered practically-identical tiling functionality in Firefox (i.e., tab tiling within one window).
It still exists, but was broken when Firefox moved to manifest. Now it tries to replicate the behavior with individual windows instead, which feels awful to use.
There's a Firefox fork called Floorp that purportedly has Vivaldi-like tiling, but after a week with it, I couldn't figure out how to enable it. Plus, it's in its early stages and some of the users are vocally anti-Vivaldi (more specifically, anti-Floorp-becoming-Vivaldi-on-Firefox) so who knows—all those features might get stripped off down the line anyway.