I fail to see any upside from the bank forecasting a long recession. It's hard to believe it's a high accuracy forecast; the central tenant of economies is that they usually do what you don't expect.

Agreed with everything else you said. "This is not going to be a quick fix, we have some idea of how bad it'll get, and it's not as bad as the 80s. Please don't panic" is perfect phrasing. But saying it will be the longest recorded absolutely is fear mongering, and partly self-fulfilling.