I think that is going out on a limb. Given that the vast bulk of pax on a MIA-PHL flight are connecting in PHL, this would've been big news since an hour's delay would cause many (if not most) of them to misconnect.

I cannot fathom a pilot's announcement explaining this unusual diversion to a nearby city shortly after takeoff wouldn't have been heard by every passenger on that flight. When I've been on flights where irrops were announced, audible groans could be heard and pax immediately begain talking with each other about it. Pax also asked FAs countless questions about misconnecting, etc, etc, etc. IRROPS for most pax is a big deal.

Moreover, I would've expected the GA's in MIA to encourage quick and early boardings so that the planes could have a shot at as early a pushback as possible given the known delay of diverting to FLL before continuing.

I wasn't there, so I'm just going on the OP's version here. All I'm saying is that, if (as the OP outlines) pax were not aware of the diversion until it was occurring, that's just unacceptable for 100 reasons.