Trickling details out about something still taking shape leads to expectation problems, and perhaps less favorable coverage (in magnitude and tone) at intended launch time.
It's too big to hide the fact that someone is doing something, so by making obvious efforts to keep it secret, they prompt people to look into it, find clues that its tied back to Google, and give more press to the effort than it would have gotten if Google had issued a press release about building floating data centers.
Perhaps also for legal reasons. For example larger companies use shell companies for purposes of reduced liability.
Or perhaps the tech/approach is untested, and they want to do some preliminary tests/dry runs before putting their branding all over it.
These security measures are pretty tame in the grand scheme of things though. I think if google did want to completely mask their involvement, they would have at least issued different badges and credit cards to their employees on site.