Because Google (not Microsoft) released a project under the BSD license (an open source license) but "everyone on Lemmy" doesn't think it's open source, therefore a hosting site owned by Microsoft (not Google) is not "open source."
I'm not even sure what is meant by GitHub being "open source." It's a hosting provider, not an actual piece of software. The site itself doesn't have a source license. The individual repositories can have licenses, which can be whatever the user who created the repository sets it to be - including open source licenses. Do you mean GitHub Desktop? Microsoft released that under the MIT license. And you don't need GitHub Desktop to use GitHub anyway.