Thats a 360 in this picture. A 420 spans all the way front-to-back, leaves no space for the back fan.
The 30mm rad + 25mm fans limitation is for 420mm (140mm wide) radiators on top of a stock Fractal Define S or R5.
There is no limit on thick 360 (120mm wide) rads, as long as your ram isnt super-high.
You can also easily mod for a thick 420mm like here: