The auto-demolish feature is the only reason you must build a house. Such a time limit did not exist before the auto-demolish was implemented and since then no other patch notes have mentioned a need to build house on a plot sooner than within 45 days.

Resetting the auto-demolish timer requires entering the house so it wasn't really necessary to spell out the need to build one. I guess SE wanted to further clarify how the feature works.