And also if I remember the rules for Mk Wii on Wiilink WFC aren’t that strict (, for example the points can go over 9999 there).
Other than that there is also no real reason for me to use it.
They weren’t paid to use the service, WiiLink WFC is simply better than Wiimmfi. Developers prefer using newer, open source services to old, closed source ones.
In the case of Retro Rewind, they use WiiLink WFC as it allows them to self-host their server for it, so they can let people join WFC without having a real NAND, which Wiimmfi would not let them do due to its closed-source nature.