How is it possible?

[Zarzak]

It's possible because it takes two days to get a class/job to 50 and another two days to get it to i90+

^^^^ This. And in a couple weeks it will take another couple days of playing to get 105+.

Already going to start having to buy non BiS slots for classes. (war, monk, smn, blm, and sch)

How do you get a class/job to 50 in just 2 days? I'm struggling because I want to lvl WHM and PGL and that info would be awesome to have, or it's just with FATE grinding?

Make sure your completing as many challenge logs as you can on the character. Do all your guildhests once. Do your hunt logs.

The monk will take longer due to crappy queue times but the WHM... You should get near instant queues and be getting a couple levels an hour except during the slow levels (mid 30s the dungeons are few and far between and kinda suck for exp)