I'm not sure my experiments have proven that. They've clearly shown that freezer temperatures are much better than room temperature, no matter whether the nic base is in glass or PET (#1 recycling code) plastic. PET is the only kind of plastic I've looked at because it's the only kind that's clear and thus allows periodic inspection of the samples without opening the bottles.
The thing is, most vendors who ship nic base in plastic don't use PET bottles. They use HDPE (#2 recycling code) plastic.
Now here's the catch: At the same thickness, HDPE is roughly 50 times as permeable to oxygen as PET.
Personally, I would be OK with keeping nic base in the freezer for a few years in PET plastic. I've seen little difference between PET plastic and glass over my now almost three years of study when stored in the freezer. At room temperature, the difference might be considered noticeable, but it still isn't night and day.
Unfortunately, I have no empirical data at all regarding HDPE. Given what the literature says about its permeability to oxygen, I would not recommend leaving nic base in HDPE bottles longer than absolutely necessary.