"Submissions require an export to Windows, macOS, and Linux. In lieu of submitting 3 OS versions, an HTML5 export is acceptable. Therefore, you must export to all 3 operating systems AND/OR upload as HTML5. Please see under Links below on how to Export your game. Submissions not including this will be removed"

Does that means that, if I provide a windows and an HTML5 builds, my game will not be considered for the jam?

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)