That’s the point. They want to stop people bypassing ads by using alternative front ends. If they succeed with server side ads, then it’s going to be difficult to block ads. Maybe not impossible, but difficult.
It’s maybe doable, but figuring out where ads start and end in a video file isn’t necessarily trivial. The app must know what to look for, and YouTube might try to obfuscate it to make it harder.
Database of ad timestamps like sponsorblock only works if ads happens at the same timestamps (and are of equal length). This is not necessarily the case.
The only reliable way I can come up with is a database of ads to look for, but that can be huge to accommodate for all possible ads. There’s also risk of false positives (risk of skipping video when there are no ad).