I figure the ancestors of those Klingons came from sometime between where Star Trek Discovery takes place and where Star Trek 2 takes place. The Klingon appearance has changed multiple times in their history by the time their ancestors began their journey to the Delta Quadrent. A half Klingon on TNG looks most similar to the pure Klingons of Star Trek 3 to Star Trek 6 and yet T'Greth knew she was not fully Klingon anyway. Any guesses on what kind of Klingon a half human/half Klingon would look like on Star Trek Discovery? Also how did those Klingons on VOY "Prophecy" know what a part Klingon looked like?T'GRETH: Look at her ridges. Morak was right. Your blood is not pure.
TORRES: No, my father was human.
T'GRETH: Couldn't you see this for yourself?
KOHLAR: Yes, but
T'GRETH: Why didn't you tell us?
KOHLAR: It wasn't important.
T'GRETH: Not important? We destroyed our ship because you said she was the mother of the Kuvah'Magh.
KOHLAR: Show me where it's written that the Kuvah'Magh must have pure Klingon blood.
T'GRETH: The Prophecy wouldn't lead us to a mongrel child.
^Albino Klingon from Star Trek Discovery