It is true that Cellebrite blog post looks like an amateur work. However, an important context is missing from signal' blog post:

Cellebrite specializes in breaking and extracting data from encrypted partitions, and (this is the important part) extracting keys from the secure keystore (Qualcomm/exynos).

From Cellebrite point of view, the data and keystore are already "given", all the remains is "breaking" the app encryption scheme, which in signal's case is trivial.