Is it true that one could eat high potassium foods on dialysis the first hour as all potassium is removed?

NO! It takes several hours for food to be broken down and travel to the bloodstream where it can be removed by dialysis.
Really, food should not be taken while on the machine because blood shunts to your stomach to digest the food. This can result in a drop in blood pressure.

Blanket no food policies are hard to swallow. Every patient is different; some with blood pressure problems should avoid eating large quantities of food during treatment but a stable patient, can safely enjoy food while dialyzing, as evidenced by food policies of European units. I would submit that blanket no food policies are evidence of patients being treated as a group rather then as individuals.
As far as eating a high phosphorous food while dialyzing, I would speculate that it also has more to do with the unit then the patient. If you're at unit that allows long runs, 4.5 hours or more, and uses baths calibrated for individuals then you could safely enjoy a bit of banana or orange, again as evidenced by policies at European units I have visited. If, however, you are treated at a unit that runs their entire patient population for 3 or 3.5 hours and uses one bath for all patients then you should continue your renal diet at all times.