Exactly.A 31% vote share on a turnout of approx 35% (which equates to approx 11% of the total electorate). I doubt that Reform would get 31% on a turnout of over 60%.
Also, there are certain types of people who like a 'protest vote' during local elections. People who want to send a message to the Government or Tory party and don't mind some of their councillors getting binned off. When it comes to the serious business of a general election in 4 years time, I'd suggest those that lent their votes to Reform will be back with the Tories.