the drop system and code runs on a valve server, it randomly generates your next drop time and stores it. You can't access this information as its in a secure separate server. Using the same method for golden wrenches having a 55 min schedule and then adding/subtracting a random amount of time and storing the next drop time is trivial. Yes a little bit more work but not much, the wrench times could have been leaked 1 wrench in advance but it would have been significantly more difficult for a small amount of work. It wouldn't have been much more on top of that to have completely unknown drop times conforming to a rough schedule. A few hours work max, unless this is the bug in robins code he was talking about when the first wrenchs dropped and he abandoned it in favour of a fixed schedule do to running out of time
The current code however is designed to run for each user independently, while the wrenches work for everybody, so they might have to completely rewrite that.
While that might be possible, it's quite a stretch, just to prevent employees from leaking the dates, what would still be possible as you noted. It really wouldn't have made it that much harder, it just would've meant the leak and the action have to be within 55 (or whatever) minutes, more than enough to launch TF2 and ready your prepared scripts.
And again, for what gain? Valve pretty certainly didn't expect their employees to do such a breach of trust.