I’m not very educated on the Fair Source stuff but the idea is that you create source available software which will after some time become Open Source. So I guess their idea is that if you use AGPL, people cannot do that. AGPL means nobody else can make Fair Source software from your work. AGPL is a good license, it just does not work with their

[new software] -> [source available] -> [FOSS after a while] -> [new software made from the now FOSS software]

loop.