It's just that we didn't actually get to see the combat. In "The Siege of AR-558", the Starfleet fighting force has already been ground down to nothing much, in months upon months of fighting: all they have left is their rifles, plus eventually a few confiscated mines. Even if combat robots were standard gear at the time, those would probably be among the casualties at that stage of the fighting (why hold them in reserve, I mean?). In "Nor a Battle to the Strong", action has already ceased when Jake finds the wounded Lieutenant, and we can't tell if his platoon had any combat robots when they evacuated on that hopper of theirs.
It's possible that if Discovery features infantry action at all, it might be the other end of it, the deployment of combat-ready troops from the ship. That's where we could theoretically see fighting robots, v ehicles, indirect fire weaponry, the works. Or then not.
Timo Saloniemi