I do not see why to limit would be 512 GB.
If it were 2GB, 32GB, 2TB or 128TB I would say it would actually be a possible real limit.
In case of 2GB it supports only SD cards.
In case of 32GB it supports up to SDHC cards.
In case of 2TB it supports up to SDXC cards.
In case of 128TB it supports up to SDUC cards.
My guess is that it was tested up to a 512 GB microSD card, but would at least support all SDXC cards.