Type names like long, short, char (and the oh-so-dear long long) are ancient cruft which shouldn't be used in a new programming language. They are confusing (as in, the answer to question: how many bits are there in your int, long, short depends on both arch and compiler) and since 70s we discovered it the hard way that they're non-extensible (multiple times, actually!). Similar for the fate of char in the post-ASCII world. C still keeps them for backward compatibility, but types like uint32_t defined in stdint.h is becoming common practice for new code.
The choices are also not compatible with C or Go, adding further to the confusion (Muon's int is 32-bit and long is 64-bit on all archs!), so even for C/C++ and Go programmers, the nostalgia turns into a pitfall. Having no familiarity with C# (not much of a Windows user since late-90s, and C# virtually doesn't exist for Linux/macOS people which Muon claims to be targeting), I looked it up, and it seems it's based on C#'s distorted and incompatible adoption of C89's data types.
Go applies the same custom to float/double as float32/float64 which makes it uniform. Go also defines int to be the size of the native integer register size on the target arch (which I like because this is what int supposed to mean originally, this was the common understanding when porting between C and assembly, although some may disagree after ~50 years of weird adaptations of it) which unifies uint and size_t, rendering size_t obsolete.
Ironically, Muon makes use of explicit number-of-bits suffix for one data type: bool32 (and there's no bool16 or bool64). I don't even know why one might have it as a basic language type.
I really hope he adopts a naming convention that is similar to C99's stdint.h or Go (which the author claims to be inspired by), or their shorter cousins u8, u16, ... used in Linux source code.