Checking the sources cited in the wikipedia page, it seems that there may be actual justification for not saying "second wealthiest"? The sentence in question specifically links to a 2015 Reserve Bank of India annual report about percentage of population below the poverty line, not state GDP. The other source, a UN/government think tank index of sustainable development goals, also says that as of 2020 Kerala places 2nd in terms of "No Poverty". So, technically, the data being referred to in this specific case was in relation to poverty rates, not wealth.

However, the second source at the same time positions Kerala in 1st place overall among all other Indian states in terms of sustainable development goals generally. So yeah, someone should maybe go and fix the wikipedia page accordingly.