Agreed. More than that, there's not much of a way to preven these kinds of attacks, period, whether in software or otherwise, if the perpetrator is some intelligence agency or such.
For threats lesser than a black op, the standard way of mitigating supply chain attacks in the civilized world is through contracts, courts, and law enforcement. I could, in theory, get a job at a local food manufacturer, and over the course of year or two, reach the point I could start adding poison to the products. But you're relatively confident that this won't happen, because should it ever did, the manufacturer will be smeared and sued and they'll be very quick to find me and hand me over to the police. That's how it works for pretty much everything; that's how trust is established at scale.
Two key components of that: having responsibility over quality/fitness for use of your product, and being able to pass the blame up to your suppliers, should you be the victim too. In other words: warranty and being an easily identifiable legal entity. Exactly the two components that Open Source development does away with. Software made by a random mix of potentially pseudoanonymous people, offered with zero warranties. This is, of course, also the reason OSS is so successful. Rapid, unstructured evolution. Can't have one without the other.
Or in short: the only way I see to properly mitigate these kinds of threats is to ditch OSS and make all software commercial again (and legally force vendors to stop with the "no warranty" clause in licensing). That doesn't seem like a worthwhile trade-off to me, though.