Yet the fact both accounts are so steeped in polemic (anti-Hebraic from the Egyptian account, and the anti-Egyptian bias in the Tanakh) testifies to the historicity of the event.
I would say just the opposite - the fact that they were so biased colors their agendas and makes their accounts less reliable. If saying that the Exodus happened gave them an opportunity to make a political point, then they would be more likely to say that the Exodus happened.
If I have two writers each heatedly laying out a case blaming the other for the breaking of a window, I may never know who broke the window, but it seems certain that a window was broken.
Okay, but unlike a window, for a mass exodus of thousands of slaves from a country, one would expect there to be some kind of record. You didn't address that.