As far as I know (I am certainly not THE specialist) this delay timer is to avoid a situation such as happened a year ago with the Iridium subsystem.
http://flightservicebureau.org/iridium-fault/After years of operation it turned out that the 24-hour message buffer maintained by the ground station would work as designed if pushed hard enough. An aircraft system had been left off for so long that when it pulled out all waiting messages after re-activation, it got at least one that was from a previous flight. Nothing in the airborne part of the FANS system was able to deduce that the message was older than a few minutes and thus operationally irrelevant. FANS was not designed with long-running slow message transfers in mind -- the thinking was near-instant, no buffering. A major oversight in FANS design, not really in Iridium's delivery mechanism. Don't trust your radios, they are too stupid.
The issue was solved by running a deletor query over Iridium's ground database every four minutes, killing off pending messages that had already been expired after three minutes by their originators, but that were not being canceled downstream by those originators (the real bug, in my opinion).
Later FANS implementations, with the "+" mark, have this latency timer that can drop and ignore messages sent by ATC more than n seconds ago. Unfortunately the system only works if both all ground stations support it (most do, all in the USA do) *AND* all airplane stations, and most don't as they don't have the right software load. Changing this software is technically not that hard but commercially very expensive, basically you need to upgrade your whole FMS and most related boxes. So few airlines do it. And if one aircraft does not have it, the problem of accidentally responding to a day-old message remains.
As far as I know, this timer therefore guards the system itself, to assure that it will not show pilots messages that originated at ATC more than n seconds ago. It is not something that, for example, warns somebody if they don't respond quick enough. That was always part of FANS. This one is just for the underlying message transport protocol.
And if PSX wants to implement this, it would need a field on the instructor FANS console "intentionally delay this message so long that it would not reach the plane in time". The plane then would drop it. The pilot would never see it, or at best receive a message that a message was dropped.
Hoppie
pretty sure about all this but not 100% so any comment welcome