DFSG is a bunch of rules that the Debian community came up with, in an attempt to create measuring points to indicate whether or not a piece of software complies with the Free Software Definition, i.e., whether the user of the software isn't being denied the 4 freedoms over the software. Don't mistake the shadow in the cavern for the real thing.
So let's see how the constraints posed by requirements of the TeX license get in the way of the 4 freedoms:
#0: run the software for any purpose: no conflict here
#1: study the source code and adapt it so that it does what you wish: no conflict here
#2: distribute the software, as you received it, whenever you wish: no conflict here
#3: improve the software, and distribute your improvements: gotta do a bit more work to rename the files, but that's about it, nothing significantly different from replacing easily-replaced logos and trademarks in any piece of Free Software.
So, no grandfathering as far as the Free Software definition is concerned. That the DFSG needed adjustments is just more evidence that it wasn't (and still isn't) equivalent to the FSD. That the DFSG was corrected, rather than insisting on a divergence, is a good sign that the people in charge took it for what it is (a set of heuristics), rather than as the essence.