If there is existing open source that doesn't contain ITAR, then that's fine because it's beyond the scope of ITAR, so ITAR doesn't apply to that scenario. [Maybe this is the case you're mentioning?]
If it is ITAR, it can't possibly be publicly available open source. [How could it be possible to have publicly-available open source software that is also restricted to being only shared with U.S. citizens?]
Of course an ITAR project could pull in publicly available open source (e.g., dependencies), but that doesn't sounds like what's being discussed here.
We can sensibly speak of tech that "would be" an ITAR violation to deliver "if it were not" open source. This is exactly the scenario under discussion. It seems very clear from the linked page that, e.g., GPS code that is released as free/open is, in fact, not restricted by ITAR.
GPS receiver systems are the classic there: Russia, China and Europe all have their own GNSS. China runs the semiconductor industry and is quite capable of producing whatever unrestricted GNSS devices they choose. Therefore why restrict US companies?
Same with satellite tech, there may be some US specific tricks but there is a reason ITAR free satellite designs already exist and are multiplying. ITAR tries to protect too much and is killing US market share by being stupidly annoying.
Whether it's open source or not is irrelevant. ITAR software cannot legally live on GitHub.com in any case -- it doesn't matter if the repos are public or private. [But a GitHub Enterprise install (self-hosted version only) can be compliant.]
I'm confused by your request for the company's stance, since it's not something up for debate... there is no room for them to take a stance on complying with the law. It's not up to GitHub at all.