Sealed light units are not airtight therefore there is no need for any holes which would increase condensation as more water vapour would be contained within the unit. The fog lights on my z4 are full of condensation because the gaskets have hardened and are no longer sealing.
Your example of the drinks bottle is stretching it….literally ?. The bottle, made from pet, is extremely thin walled at 0.25mm and will flex easily when subjected to an increase or decrease in pressure/volume. A rear light, made from 3mm PMMA, is a well known dimensionally stable polymer, it wouldn’t deform with the slight increase/decrease in pressure caused by the Sun/cold even if it was fully airtight, which it isn’t. Don’t drill holes into light units, it’s an internet lie, it will only allow more moisture heavy air into the light and more condensation, this is the problem with the mg4 light, the seals are not doing their job correctly. It could be a poor choice of material, a design or assembly error?