I feel like this is kind of the amateur-hour stuff. It's certainly dangerous, but in comparison to a lot of state-actor activities (or even committed-amateur activities), this kind of supply-chain attack is pretty blatant and easy to spot. Which doesn't mean it's easy to spot -- I just mean would be trivial to volunteer and contribute some minimal fixes and enhancements to some open source project, and then at one point smuggle in a zero-day that will basically never be detected unless someone detects the intrusion itself and then works backwards from there with a ton of time to spend on it.
If you've ever looked at the obfuscated C contest it should be obvious that this kind of thing can be made completely invisible if you know what you're doing. Some of the interactions and language features that lead to problems are basically impossible for a casual viewer to see, even if they're paying attention, and the attack surface is massive and the amount of attention that goes into checking it for weird subtle vulnerabilities is minuscule.