Here's the context after some discussion.
The investor in question is prominent, invests in a lot of companies, and owns let's say 20% of the company. I own a controlling interest in the business as the founder.
I've been out of contact lately with the investor, working on readying a product. The concern was about me as it turns out, due to the lack of communication. I play an integral role in the company and the product is my vision.
The investor had been pretty chill about requiring constant updates, and I got too relaxed about keeping that communication line where it should be. This is something at times I'm bad at, I have a tendency to bury myself in something and not come up for air for a long time.
So the request for source / server access was, in the investor's words, a precaution. Without actually knowing something was wrong, I don't know that it exactly justifies an investor having access to those assets directly, but it sheds light on the thinking. Clearly I need to re-instill confidence there.