I have not heard this! It's very interesting and if true, scary. I have been using GSE for years and haven't had any bad side effects. I use in all the time on pacifiers (cleared up thrush this way) to clean my toothbrush and then my mouth. I've cleared up ring worm and other infections so I know it works.
I would wonder if the lack of preservative isn't causing the GSE to just loose it's potency and not that the preservative is actually what is antimicrobial??
I do know that companies are not required to list all ingredients by law so it's totally possible that there are preservatives in brands that do not list them. I use Nutribiotic and they do say it's made from Cert. Organic grapefruits but not that it's organic itself.