The simple answer is: white was already completely lost; Rf4 went from a losing position to another losing position, so not really a mistake.
The longer and more technical, though also more correct answer is: consider lines 62-67 of the source file
github.com/lichess-org/lila/blob/master/modules/analyse/src/main/Advice.scala. In line 62, white's winning chances are calculated, both before and after the move Rf4. Before the move, the formula calculates that white's cpWinningChances are -0.92. After Rf4, the formula gives -0.99. Since this difference is less than 0.1, the move is not regarded as a blunder, mistake, or inaccuracy.