Windows MMBasic is quite old and hasn’t been updated. (Not sure when, or if, it will be updated, as it is apparently abandoned.) PicoMite (and PicoCalc) also doesn’t support everything available in MMBasic due to the platform constraints. The PicoMite manual (linked from here) discusses all the commands available on PicoMite/WebMite which are also available on PicoCalc. Not everything is available on PicoCalc though, since it doesn’t support any of the HDMI or VGA stuff.
There are also a few extra commands available on PicoCalc, mentioned here.
Unfortunately, there’s no really good, 100% compatible option to develop MMBasic code for PicoMite without either a Pico module and custom hardware (like people use on TheBackShed) or a Pico module inside a PicoCalc. MMBasic for Linux is probably closer to covering most of the the MMBasic language available on the PicoCalc, but it’s still going to be missing some things (like WebMite functionality) and will support some commands that don’t exist on PicoMite. It’s at least more up to date than the Windows version though. And it has provisions for “simulating” some PicoMite compatibility.
It’s a shame there’s no “emulator” or truly compatible desktop version available, but weirdly it seems like the folks on TheBackShed have no interest in such a thing and would rather just use the hardware itself.