Their match was played in November 2018. Here are the classical [1] and rapid [2] rating lists from that time. Fabiano was 12 points behind Magnus in classical, and 91 points behind him in rapid. It's all going to be relative. Obviously Fabiano is extremely good at all forms of chess, but all top players (in any sport) are going to have relative strengths/weaknesses when compared against each other.
The number of games does influece the stability of the ratings but the result is the opposite - classical ratings are more reliable because there are regular organised events while there are very few serious blitz and rapid events. Most top players only play the rapid and blitz World Championship, no other events in these time controls.
