Not quite. The guy who wrote Tor can be clearly seen to have been studying a lot of these things in the later 90s, and it was pretty much always public, but only developed after he got the Navy to get him an NSF grant. Hell, in one slide he had made back then, he was even well-aware of some of the fundamental vulnerabilities in the protocol, and the (still too expensive) mitigations for those flaws, so it can't even be said that anything was concealed.
If we go back further to the original concept of chained anonymous remailers as envisioned by Chaum 40 years ago, it gets even harder to claim something like this.
Well, not all the exit nodes by now. But many of them.
Most people are probably leery of running an exit node, because its traffic is in the clear (modulo ssh) and often connects to disparate and shady servers.
This is what I've never understood about Tor. What possible incentive does anyone have to run an exit node? Seems like nothing but a liability for some extremely awful stuff.
There is no incentive to run any node other than feeling good about it or using it for intelligence purposes. It's a volunteer thing. But to run exits you just need a host who won't kick you for every single abuse notice.
Tor was created by the feds for spies to use, then they made it public yo hide the traffic.
This is the official story that everyone has evidently forgotten.