Indeed, it happens that a subset of the freedoms ensured by the AGPL align with what the OSI wants, but that is not why the license exists. They are completely different movements.
The original comment was:
> AGPL is excellent for promoting the spirit of open source
This statement is not about the practical compatibility of the license, this is a statement about what the license stands for. The license categorically does not stand for Open Source.
AGPL is useful also for preventing commercial improvements that don't make it back into the general product.