Maul defeated Grievous once, by BFRing him with telekinesis. In the encounter where he tackled Grievous not only did he have enough leverage over Grievous to force a capture over him without resistance anyway, but he off-balanced him with a Force push before leaping on him. I don't see how either of those apply to this fight except from the fact he can throw Grievous with TK from time to time. Maul's telekinetic power increase was substantial between TPM and TCW so that should be noted.
Eeeeeeeeeeeeeeerrrr TWC is canon for economic terms.. But a bullshit official or not stay a bullshit.
And Grievous is over powering darthmaull with his 4 light-saber but lack of technic.. but if you consider 2003 TWC canon... That's mean Grievous is really well trained...
But Darth maul as also his weird light-saber (remeber at the time it wasn't commun to find those dual blade...) and a lot of technics !!!!