Although one of the meanings of trenchantly is "vigorously, energetically," which by some stretch might fit the context, I suspect the author thought that trenchant had a lot more to do with a word with a similar root: entrenched. Beyond this etymological connection, however, the two words have nothing to do with one another.
Trenchant ultimately derives from Old French trenchant, present participle of trenchier, to cut. Today, if someone is Paris wants to slice up a chicken or roast, they use the verb trancher. Thus the metaphoric cutting and incisive character of trenchant.
Entrench, though from the same root, didn't follow the same route into the language. It is a native English word derived from trench, i.e., something dug (cut) into the ground, especially for defensive purposes. An entrenched position is defensive and unmoving, "dug in" for battle.
Thus while other departments gradually changed, mortgages didn't move at all, but maintained an entrenched position, remaining the same.