Once the output is built and uglified, you will not reverse engineer it back to the source that google started with. JS projects are not not much different from any other built project. Probably closest to .Net or Java IL once uglified. Still higher than WebASM, but still not he same as source.
It has to be open source in the sense of sharing the source code, but companies take the extra step of making it open source in the sense you have a license to use it for your own website.