...buy a device...and only then be able to read the manual... not a reasonable suggestion...
Like all manufacturers of sophisticated audio gear, TASCAM makes available online the user manual, as well as OS compatibility information, the most current drivers, the most current firmware, as well as the most current list of tested/certified SD cards for each of their products.
I wouldn't presume to speak for all audio engineers, but before plunking down
my hard earned ducats, my due diligence includes going first to the manufacturer's web site to verify system/driver information and obtain/download/read the manual to assure the product will meet all my needs.
Read the manual; check the firmware; check the list, buy a certified card. SOP for me.
I keep my firmware up to date, use only tested/certified SD cards, and have been problem-free for the more than twelve years I've used TASCAM digital multitrack recorders.
I'm merely passing along the information found on the TASCAM web site and in the owner manual; and the experiences, across product lines, over many years, of the members of this forum community (which a forum search using "SD card" turns up easily). It is what it is. I really don't care why.
... says it supports a 128 GB SDXC card ...
I take your point that the generalized SD card info in the product spec sheet would benefit from a caveat.
Perhaps TASCAM assumes users of their complex multitrack gear will take the time to read through the manual before first use and see the caveat up front in the "Introduction/SD Card" section.
TASCAM has been producing sophisticated digital multitrack gear a very long time. Perhaps they've found (and empirical evidence on this site suggests strongly) that the complexities of their multitracking products strain some SD cards to the point of failure.
Perhaps you can take that up, and the why/wherefore, with TASCAM; come back and share with the community; and in contributing to the community in a positive way, expand the collective knowledge base.