We saw that if Arcann is ready, he can resist at least a part of Valkorion's power. Obviously the power he utilized through HoT was weakened in itself, and it was done via a vessel, so even more weak, but it must have been a substantial part of his power.
Then add in Revan, who already could stand up somewhat to the power of Vitiate, obviously novel Vitiate was a far lesser being, but also Revan was far from peak too, just released after inhumanly long imprisonment, on a Dark Side nexus, and after a long fight and hiding eating fecals 🙂 🙂 🙂
The Dread Masters may suck in a direct combat, but if they go into the fight prepared to know who will they oppose, and with Revan and Arcann there, they can provide enough of an opportunity. Even Revan and Arcann together won't stand up to Valkorion for long with that distance (20 meters is just too much given Valkorion's skillset), but there will be nearly a dozen of people, and most of them (DMs) in a naturally perfect coordination and a crowd that large is a huge factor in itself. Valkorion can't defend on every front at once, as superior as he is.
The Dread Masters together were inferior to SWTOR Vitiate, who was considerably weaker than Valkorion. Revan was stated in SoR to stand no chance against Vitiate. Add in Arcann, who resisted his fathers power when not only weakened but channeling his power through an extremely inferior vessel. Not seeing how team pulls any wins. Together they'd stand a solid chance, but still'd go down hard enviably against a fully focused and unhindered Valkorion.