I’ve tested this in the Editor with triggers, giving one player both Celerity and Monstrous Rage at the same time. They definitely stack - watching a Colossus swing that fast was hilarious - but Titans don’t get the attack speed.
I get the impression this is intentional. Titans do so much damage per hit that the only thing keeping it in line for balance is the very slow attack speed. Giving it an attack speed buff would be a wildly outsized DPS increase.
If it IS intentional, I would at least appreciate an edit to the respective upgrades’ tooltips making that clear.