As I see it, the argument is as follows:

1. If investors were pricing quantum risk correctly, they would need deep technical understanding or close communication with experts who do.
2. Deep technical understanding would lead to accurate pricing of other technical risks to Bitcoin.
3. I do not believe those other technical risks are accurately priced in. I would wager most investors are unaware of them.
4. Therefore, quantum risk is likely not accurately priced in either.

Defining "accurate price" would be an interesting exercise. It's a convenient way to call the market irrational though.

To be familiar enough with the technical risk quantum poses to bitcoin, investors would need to be extremely sophisticated. If they understand quantum computing risk, they would necessarily understand  other, less arcane risks to Bitcoin that are absolutely possible today.

There are even financial incentives to execute some of these attacks on Bitcoin. The most crippling one, a poison block attack, didn’t even have anyone monitoring. Perhaps this was due to complacency or, quite frankly, denial that anyone would attempt one. (A “poison block” is a block that basically makes it impossible for other block producers to validate your block in any reasonable timeframe; you can learn more about it in this talk from last year’s OPNEXT).

The omnibus Great Consensus Cleanup proposal that would mitigate a few of these attacks has been sidelined for half a decade, only recently having been picked back up.