the underside of the wing is not an overly sensitive area to the performance of the wing section
I speculate entirely about the damage to the wing resulting from the wing tip float being ripped off (let alone probably the leading edge of the wing also striking the tree). It is possible that when the wing tip float ripped off, it tore skin from the underside of the wing too. It would not take too much ripped wing skin, peeled back and flapping, to cause immense drag, which is well out the wing as well. I cannot speak for the CL215/415, as I do not know it's structure, but I know that other smaller amphibious flying boats are designed such that you can rip off a wingtip float without damaging the wing. I have found this to be true on mine
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In any case, that was a remarkably short time for the crew to reassess the possibility that control might be maintained....