My take on it is Guardiola is really just that stubborn. Making a sub, in his mind, likely compromised the way he thought they would win the game and that is the absolute #1 thing for him, never deviating from his idea and having 100% total devoted belief in it.
Does make him look a bit silly when stuff like this happens though. He's undoubtedly a genius but his stubbornness is getting increasingly obvious, and to the detriment of getting results.