I think there's a bit of confusion about what exactly Google is doing. IIUC they use (and develop) gerrit.
Effectively this workflow means everyone is working on the same branch and the first commit to pass review gets in. The next guy will have to rebase.
In the end one wins as you said, but the level of detail is rebasing individual commits, not merging entire branches.
So this ends up being the age-old rebase vs merge discussion.
Someone’s changes get committed first. That’s a business decision, not a code tooling one. Second pr has to adjust. Same on both mono and poly repo, just using different words.
At least branches let you have the choice, which cannot be said for branchless.