Right on. But it's as close as anyone has come to "shipping the source code along with the CD". So even though Id did it after it generated a lot of revenue, they still did it, which is extremely rare.
Also, one reason that Id's most recent engine (Id Tech 5) is closed and licensed is because of its brand-new technology: MegaTexture. As of now, no other engine has implemented that feature (or the engine isn't production-ready like Id Tech 5 is). So yeah, if you invent paradigm-shifting new technology, then it is probably a bad idea to immediately open-source it. :)
I agree, it's incredibly generous of them and very helpful for the next generation of game developers to be able to read and use "real" code like this.
And I think that more or less all their engines have been revolutionary when they were released, not just this one!
Giving something away -after- it has earned you a billion is less risky than doing it -before- you've earned the money.