So, Smilodon fatalis weighs 280 kg at its max while T. walkeri already surpasses 300 kg.

I could say that S. fatalis has better weaponry but the big terror bird could kick like seriemas. My money's on the big kitty but it could really go either way

Marginal weight advantage means nothing for Titanis when cats are adapted to taking down prey several times their size.

Precisely

Tbf, Titanis was hunting the exact same things Smilodon was were their ranges overlapped.

Yeah, but birdie hit them in the nape with his beak until they died, which it obviously won't be able to do once Smilodon pins him to the ground and bites his throat out.

Terror birds didn't kill things by hitting them with their beak, they killed their prey by slicing their with their beak, similar to Smilodon.

^^ Bold of you to assume that a Smilodon can easily pin a giant, heavily-built hypercarnivorous bird with an axe for a head and powerful legs/talons to the ground.

That thing can pin down multi-ton herbivores to the ground after all.

No

If this was S. gracilis, it'd be an easy one for Titanis, but this one I feel could go either way depending on whether S. fatalis was generally solitary or not.

Too many variables to consider for a fight than just who's heavier, for sure, but Titanis also has a significant reach and height advantage, which may be enough to bluff the cat into turning the other way.