Technical breakdown of runway separation occurs when the landing aircraft crosses the threshold to land. I would initiate a go around prior to crossing the threshold if I hadn't received a clearance. It's the difference between an occurrence and an incident. If you've already crossed the threshold, the damage is done (although perhaps not literally).
Controllers can issue landing clearances when there is reasonable assurance that separation will exist at the time the landing aircraft crosses the threshold.
In this case it was obviously going to be tight and in my experience controllers tend to wait, unsure of whether it will work and issue the clearance too late. If it's true the 50ft call was heard with the read back, that's too late in my opinion. 100' - 200' is your last chance to make the call.