Maxim is legit, but not much information on the internet (in English). As mentioned above, there are some Rickson Gracie black belts in the Netherlands (Maxim and Harold Harder) the EGJJF
https://www.egjjf.com/en/rickson-gracie-seminar/ is based in the Netherlands for a reason, there is a healthy Rickson Gracie jiu-jitsu lineage here with black belts like Michel Verhoeven and Raoul Hiwat as the most prominent. I do find that quite often Rickson Gracie fighters are not as active in BJJ competitions as they focus on self-defense rather than sport jiu-jitsu. This means they are not as easy to find online (no competition results and so on). If you have any questions surrounding the legitimacy of Maxim's black belt or level, I would recommend finding him in his dojo and having a chat .. :
http://www.bjj-purmerend.nl/grappling-mixed-martial-arts/
and I doubt the Last Emperor would waste his time on trianing with anything less then the best trainers out there..