My theory is that, if seeds are very dry to start with, plastic seed packets are not that bad. They do "leak" humidity and oxygen in both directions.
If you start with seeds that are only "moderately air dry", like 50% RH, then put those in plastic, they might metabolize enough and release enough humidity that doesn't diffuse out through the plastic zipper fast enough.
Then the seeds might go from 50% RH to 80% RH, which could lead to mold.
Better to dry seeds very well before packaging. Like 15-30% RH.
That said, I usually do store some desiccants in the tubs where I keep plastic seed packets.
