Well, then it provides so little they could have not done anything at all as well. Useful stacked PRs would be something that uses AI to split my large branch/PR into smaller PRs for upstream to review. Claude does that for me.
They've provided a skill with their stacking CLI to provide guidance to any coding agent and the Copilot harness now ships with a pr-stack command to do exactly what you're saying for PR splitting FYI: https://twitter.com/_JeremyMoseley/status/208219039710799469...
It's not "so little". I've wanted this for ages. Trying to submit a chain of PRs is a horrible experience without support for it. This is nothing to do with AI.