AGPL is an open source license as per OSI definition which you yourself point to!
You don't have to take my word for it, just check out their own site:
https://opensource.org/licenses/AGPL-3.0
The licenses you are talking about, such as MIT or Apache, are called _permissive licenses_ whereas AGPL is a _copyleft license_. However, both types of licenses can be open source licenses, if they fill the OSI requirements.