OK, seen about a dozen posts today that start with:
"When #Threads federates with the fediverse..."
They go on to imagine some wonderful or awful scenario where the floodgates are open and #Threads becomes another #ActivityPub app like #CalcKey or #Plemora.
Maybe.... but what you are envisioning is really very unlikely. Threads is entirely algorithm driven. How's that going to work? Plus they have no interest in letting their captured users escape.
Be careful with these assumptions.