"We have not seen him since February."
Translation:Od února jsme ho neviděli.
Not sure where that rule comes from. Never heard of it.
But "Nevidíme ho od února" which would be present tense, makes no sense in Czech at all.
Present perfect is translated using either past or present depending on situation. To use present here, you would have to use different form of the verb.
Nevídáme ho od února. But that suggests that we used to regularly see him and that has not happened since February. The sentence in this exercise only means we have not seen him since February but it is not clear if we saw him any other time but that one time in February and never before and never after.