I think it makes sense that you can also provide downloadable executables next to your web build, even though you cannot compile for mac (we're doing the same). A lot of people cannot afford a mac. I'm sure the organisers understand. I think you can just ask on their discord if you want to be sure.
PS: if you use 4.3 your game will probably do fine in the browser (just don't use too many shaders and particles, they work well, but cause slow loading times)