I would agree. I saw it as an additional feature, e.g. allowing to drop accidentally picked up line (from unrestored button) just by hitting the hold as a button restoration. Apparently there is no need to place a bare dial tone (with no incoming call or any dialed number) on hold. 400H effectively prevents such “false hold” situation.

400H has another advantage of being more “solid state”, having only a single mercury wetted contacts reed relay. So it is “silent” in operation with no audible clicks like from other relay-based line cards. This is only an advantage when the KSU is placed in the same room as phones, not in a separate closet. One can still hear the interruptor running, but only when hold or ringing occurs.

A big disadvantage of 400H is its zero repairability due to several custom ICs used for handling the logic. So if the card is troublesome it can only be thrown away rather than repaired.