I guess they're talking about GDP per capita? The UK's GDP per capita was ~$42,000 in 2019, which was lower than all the US states + DC, except for Mississippi according to [0].

It's pretty misleading to frame this as though the UK is drastically lower than any other comparable countries though. The UK's GDP per capita is similar to Canada, France, Israel, Japan. And that doesn't even get into whether GDP is actually a realistic measure of how "poor" a country's people are (c.f: Ireland, which has a much higher GDP per capita than the US, but similar material conditions to the UK).

0: https://en.wikipedia.org/wiki/List_of_U.S._states_and_territ...