Maltz did say that Klingons didn't use sarcophagi anymore.

Also, I think it's important to remember that real cultures aren't totally monolithic and homogeneous, even if that's how they like to present themselves. In existing show canon, there are a number of variations on Klingon funerary practices. There's the {Heghtay}, where you hold the eyes open and scream. There's the {'aQvoH}, where you stand watch over the corpse. A "Klingon mummification glyph" is referenced in Star Trek 4, so clearly they did that at some point. And nobody held open Chancellor Gorkon's eyes after his assassination. It's not a major contradiction, but rather simply a sign that Klingons (much like, say, humans in Starfleet) are more diverse than they seem (or want to seem) to outsiders. MO has touched on this idea occasionally: acknowledging that an empire could have room for another language like Klingonaase (see http://klingonska.org/canon/1996-08-rt.txt), the opening to The Klingon Way describing how seemingly-contradictory proverbs can still fit in the same culture, and the whole section in KGT about the myth of Klingon conformity.