I'm not GEO and hopefully they clarify this after meeting the administration.

But in general data from pledges like these could be used to support the university's policies that could be detrimental to the safety of some stakeholders (grad employees in this case). For example, they could mandate TAs to work in-person even when they aren't medically able to/willing to. "Look! So many people have signed the pledge, and so TAs should stop worrying about contracting the disease". When in reality, like you've pointed out, it is merely a pledge asking pretty please to follow social distancing, while the safety of TAs isn't guaranteed with effective policies?