Measuring it in a foreign currency doesn’t without accounting for that is just answering “how many iphones can country X afford” (handwaving away the nuances of what GDP measures with “afford”) which might be a statistic you care about but probably isn’t (and isn’t for the point that was being made).
Same can be said about paying 100x more on prescription drugs and counting that as GDP. Also having the most expensive french fries doesn't make it a bigger economy just because iphones are cheaper.
This is actually valuable for drug exporters (and yet another reason Trump can bully countries around - US market is the most lucrative market there is).
It could be said that the US was the largest economy when it was 10x the next by most measures, because it was clearly just better than the alternatives. Not so much now (their model still turns out the best per-capita numbers for a large country though).
The more important measure is really who has the economy best configured to win a war. I expect China to be able to out-produce the US at mass cheap components, especially for drones, so for military purposes I'd expect China to have the long term edge in the sort of moderate intensity conflict we're seeing around the globe. Hard to be confident though, and the exact nature of such a war is unclear because diplomacy would be a huge factor.