From what I recall, the Ant-Man and the Wasp mechanic of entering the quantum realm did require an outside for to return the traveler back to the ordinary world. Hence why Janet and Scott could be trapped there.
However, as you say, clearly the time platform does not exist in the past and so is not necessary to exist to travel to it, and as the changes Tony and Steve make to make an additional jump have not been accounted for by the time platform, so clearly the time platform does not limit where they can travel to or from.
He could have gone to the alternate reality Tony or he could have held onto it and used it later after she died. I prefer the latter because it requires the fewest additions to the story, but I can see reasons to go with the former. However, it should be kept in mind that Steve was always going to make multiple jumps to multiple time periods, so he would have to have some degree of control to determine when he needs to make the next jump, and alternative to not make the jump or choose to make the jump at a much later point in time.