The point he makes (which is a good one) is that some think lower taxes, especially capgains, are the answer to everything. They're not. What's more, it's a veiled opportunistic argument at best. How so? Those advocating for it, aren't really interested in increasing aggregate consumption (which is what they argue), they're interested in keeping more of their money. Don't believe me? Ask anyone who is parroting this supply side argument, when IS a good time to raise taxes and see what their answer is.
The informed wonk, academic or well informed citizen will tell you during war (the simpleton will show their colors by arguing for a flat tax or saying no increases ever...when you hear this stop the conversation immediately because you're talking to an ideologue who will only waste your time repeating fantastical free market memes they've heard somebody else talk about on tv.)
With war being a valid answer, ask this informed person why we didn't raise taxes during the Afghanistan war and then ask them why we didn't raise them during the Iraq war, then ask them how is it we've been in two wars for over 5 years, at a cost of over $7B/mo. and haven't raised taxes. Then ask them again, so when is a good time to raise taxes? If they say during war, remind them that we still have two on.