Given that’s on the cab floor, I wouldn’t really worry about it. Check the paint is bonded well to the metal and if it is, I’d scuff it and prime over it. Or just scuff and paint. Don’t worry about the primer sticking to the rust oleum, make sure the rust oleum is stuck to the metal,IMO. Fwiw, I’ve checked the “bond” of the primer to the 2 types of existing primer, existing paint, bare metal, slightly rusty metal, previously painted unprepped areas and it all appears to stick well. Got a bunch of overspray on the old undercoating that was just washed clean and it sticks to that as well. If this is a truck that the floor boards may get soaked wet forever again with wet carpet or whatever, I’d consider a quick sand back to mostly bare metal just so I wasn’t relying on $4 a can paint to stick to bare metal, but most likely I’d not even waste much primer inside the cab and just shoot it with top coat. Paint will stick to properly prepped paint, have painted a few vehicles without using any primer save for covering bare metal or as filler.