[rybosworld]

> in fact if you plug it into an engine, it shows an equal evaluation

This isn't true - that move takes the game from dead equal, to +0.7 (70 centipawns favor for Boris).

It isn't a total blunder but it's objectively a poor move.

[dmurray]

A better engine or running the engine deeper will show 0.0 (or you need to show there are alternatives for White before 40... f4?!)

Though this may be a situation where the weaker engine is actually more helpful in assessing the practical situation: +0.7 is a better reflection of the relative chances in a human game than 0.0.