Sort-of. This is definitely true if you assume that time discounting is fundamental. It's also definitely untrue if you treat time discounting as a kludge to accommodate uncertainty and also see estimates of climate costs as already containing that uncertainty. https://twitter.com/PTetlock/status/1170789498109747201 …
iirc the climate economists who assume a near-zero discount rate already take account of catastrophic risks and it doesn't raise discount rates much, as you said. The real argument for having discount rates is that it's truer to human psychology than not.