I updated my original post with examples.

Awesome, I'll dissect them and show you exactly where you're drawing the wrong conclusion ;)

I wrote an article talking about design by contract and why it's better than TDD here

Nobody is advocating for TDD or disagreeing with that. Having unit tests != TDD.

https://github.com/mempko/henhouse

So far as I can see, you're literally the only developer here -- which exactly illustrates my point. You can ignore a LOT of good engineering practices and be sloppy about a ton of things if you're the only developer (or one of a handful of developers), because hardly anything ever changes underneath you or without your knowledge. (Source: I've done it too.)

See SQLite (uses assertions heavily)

SQLite has like... 3 developers? The vast majority, again, being 1 person. (I didn't even bother verifying your claim that they don't have unit tests, FWIW.)

.NET framework from Microsoft

.NET absolutely has unit tests, here's one example: https://github.com/dotnet/runtime/blob/ebabdf94973a90b833925...

Many of the C++ Boost libraries

All the ones I recall ever seeing have unit tests... which "many" don't? Here's one that does: https://github.com/boostorg/regex/blob/develop/test/regress/...

Parts of Rust

Again, you're gonna have to cite what you're talking about because Rust definitely has unit tests: https://github.com/rust-lang/rust/blob/master/library/core/t...

Parts of Qt. The Vala programming language, Ada GNAT... and many others.

I'm not gonna keep digging up their unit tests, you (hopefully) get the point above.

I also included this research paper from Microsoft that shows a clear advantage of contracts and code quality

As I said above, nobody is arguing against contracts! A paper showing they're awesome doesn't mean they're substitutes for unit tests in every situation. Your paper only mentions the phrase "unit test" twice, and neither of them is saying DbC substitutes for them.