This statement is highly misleading. Fossil fuels are exempted, but only for a limited number of circumstances. The wikipedia article you linked only mentions fossil fuels being used for electricity generation being exempted. Furthermore, you failed to quote the rest of the paragraph, which mentions fossil fuels make up a minor part of electricity generation in Washington.
"However the majority of electricity generation in Washington state is derived from renewable sources. [...] The same analysis further indicated that only 4% of Washington's energy comes from burning coal, [...] Natural gas currently accounts for only 10% of Washington's energy generation, according to the Washington Post analysis. "
>Also, midterms have a much lower voter turnout and in Washington it was 58.9% in 2018 vs 75.71% in 2020 [2]
Except that the 2018 ballot measure wasn't the first time Washington tried instituting a carbon tax. They tried 2 years before (not a mid-term year) and that failed even harder, 41% to 59%.