The Heat aren't trading Robinson for Aldridge. The only expendable, notable young player they have is Nunn. Obviously, he doesn't make sense for the Spurs, so bringing in a third team and getting something that does (earlyish 2nd or commensurate youngish player is probably his value) is how this could make theoretical sense.
Realistically, the Spurs aren't doing that though (or anything) unless Aldridge requests a trade.
Some of these takes, while typical, are simultaneously astonishing. Why would the Magic or Kings have interest in Aldridge (to play a position he hasn't been able to in years, no less), why would the Magic trade Vucevic for no notable Spurs youth/picks and why would the Kings feel compelled to trade Barnes? None of this makes sense.