If it was the FTS wouldn't the flight control systems send a message back to the ground saying "things are going sideways here, FTS Activated"

Maybe it did, or is it public that it didn't? A possible sequence (very typical in rocket failures) is: fire, engine failure(s), loss of control, rupture due to aero forces or FTS activation, explosion due to propellant mixture. Not all of these have to happen, but it's a typical progression. Before the days of AFTS the FTS activation would be pretty delayed.

Eh I'm thinking more it was a reentry explosion from pressurized tanks. Engines had failed a while before then.

This is over the Bahamas. Re-entry was much further east, near Turks and Caicos Islands.

Also, if a pressurized tank is reentering, that means the FTS failed to detonate.