Reviewer of PhD thesis doesn't allow me to write in the first person singular
There is an annoying truth here: What's OK in a PhD thesis and what not largely depends on the reviewers. Some people in academia have a huge inflated ego and prefer things to be done as they suggest, and the reviewer in your case is possibly among those people. You don't want to fight with them on this kind of issue.

If the reviewer has other comments that tend to create additional work without clearly improving the thesis, you may consider exchanging the reviewer.