I'm not sure why so?

Because URL shortening is relatively trivial to implement, and instead of just doing so on their own end, they decided to rely on a third-party service.

Considering link permanence was a "founding principle", that's just unbelievably stupid. If I decide one of my "founding principles" is that I'm never going to show up at work with a dirty windshield, then I shouldn't rely on the corner gas station's squeegee and cleaning fluid.

First of all, how the links are made permanent has nothing to do with the principle that they should be made permanent.

There seemed to be two principles at play here:

1. Links should always work

2. We don't want to store any user data

#2 is a bit complicated, because although it sounds nice, it has two potential justifications:

2a: For privacy reasons, don't store any user data

2b: To avoid having to think through the implications of storing all those things ourselves

I'm not sure how much each played into their thinking; possibly because of a lack of clarity, 2a sounded nice and 2b was the real motivation.

I'd say 2a is a reasonable aspiration; but using a link shortener changed it from "don't store any user data" to "store the user data somewhere we can't easily get at it", which isn't the same thing.

2b, when stated more clearly, is obviously just taking on technical debt and adding dependencies which may come back to bite you -- as it did.

You're always relying on someone else, no matter what you do.

Also, "they" is the person you are replying to.