as accordingly they would likely support the VSCode extensions, that means that the existing VSCode extensions (e.g. arduino extension) are likely to work in theia as well.
as theia is late to the party, they will face an uphill task if they want to do their own extensions, i'd guess that's the reason they moved to support VSCode extensions
actually currently i don't quite like the Arduino VSCode extension. there are apparently various problems, but i'd guess this is pretty much 'beta' software and is evolving. there are a lot of issues if you review their issue log
https://github.com/microsoft/vscode-arduino and gitter
https://gitter.im/Microsoft/vscode-arduino
but literally, among the reasons for not using the vanilla arduino IDE is due to the shortcomings.
e.g. no reference tags/symbol jumps, e.g. drill down into function, or reference jumps to structures
and i really disliked the use of temporary output folders. builds are quite large e.g. even with stm32duino core itself
and the binary is 'missing' after the build due to use of temporary folders. it is a reason i stayed with Eclipse and CDT.
makefiles and all are relevant in that context too
the thing about makefiles is that most consider it a 'unix' thing. so a cross platform solution calls for something like arduino-cli.
but literally the arduino community could have adopted other build technologies like apache ant etc. and is used to build java apps and is plain old and robust. and there are lots of new ones maven, gradle etc
https://github.com/jfklingler/gradle-arduino