If we think about it in logical terms, they could sabotage Debian by having “interests” that are suboptimal for their core demographic.
this is a similar to how euro-skeptics are the people who make the very unpopular laws inside the European Union, leading to all the negative press about the European Union. But they have to be listened to as they are democratically elected and it is a democracy.
https://www.debian.org/intro/philosophy
I think employing the project lead of systemd gives Microsoft a kind of influence that employing the packager of libjpeg-turbo wouldn't. Lennart is notorious for doing things arbitrarily, and what we are discussing here is that the Debian package maintainer for systemd is also doing things arbitrarily, and is also employed by Microsoft.
Also individuals tend to prioritize work that benefits employer interest but that doesn't mean they can do things arbitrarily. It just shifts energy and focus towards certain areas. It's not a problem unless the company employs a large fraction of Debian maintainers which Microsoft doesn't.