This isn’t some new development. Anyone who’s had regular drug testing in the last 20 years is aware of this. Clearly, you haven’t had a regular drug testing requirement for a job, parole, or any other reason.
If you had, you would know that modern tests moved the threshold of detection up because of these issues on early era drug tests, which is why this idea persists.
I won’t call it a myth, because it’s always possible a batch of food grade poppy seeds wasn’t properly processed, and that batch has unusually high alkaline contents, or that someone consumed a disgustingly large amount of poppy seed muffins, or salad dressing, a day before their test, but that would be the exception, not the rule.
Also, have you never had the poppy seed salad from costco? The dressing is in a small plastic ramekin with at most, a tablespoon of poppy seeds, but probably less.