Yes, it baffles me that the datasheets conflict with Mr.Challenger.

Anyway, after staring for a while at the TMS6100 diagrams in the Speak&Spell patents, I'm starting to think the latch timing is different for nibble mode and serial mode. See fig 20a,b,c. patent US4189779.

Nibble mode is latched on _LOW and _HIGH on fig20c. This sounds very generic but it's obvious these signals are coming from the counter PLA on fig20a.