> During the mid-to-late Pleistocene (∼1.2 million to 11,700 years before present) ... with temperatures ranging roughly between −6°C and +2°C relative to the pre-industrial mean of ∼14°C

Does this mean during mid-to-late Pleistocene it used to be -6C to +2C, or does it mean it used to be 8C to 16C?

(If the former, then how did early humanoids, and many other animals, survive such cold?)

The latter. I suggest reading https://en.wikipedia.org/wiki/Paleoclimatology#Reconstructin...

The later, "−6°C and +2°C relative to" is relative to the mean of 14.