is probably referring to fluidmax a binding for Max/MSP that we used to ship with the source code up until 1.1.11. I removed that binding from the codebase in June 2018 because
* it hasn't received any code changes since 2009,
* it used functions from private fluidsynth headers, and
* I really don't think it is fluidsynth's job to maintain bindings to various applications out there.
The fact that now, more than 2.5 years later someone finally complains, confirms to me that this binding isn't cared about much.
My best guess is that Ruben kept using fluidsynth 1.x, now he upgraded to 2.x and, expectedly, it doesn't work anymore.
If somebody is willing to maintain this binding, just grep the source code from back then, adopt it to fluidsynth's current API, get rid of all private headers, internal functions, etc., and put that binding in some repository ideally with CI pipelines for Win + Mac.