Not sure what you mean "this hasn't been linked to anyone"?
It's a bit of a dubious comparison. They play on different sides and are slightly different types of "FB".
Atal's a right back / RWB (who has been linked to us amongst others I think) who played most of the season as a RWB in a 3CB system or as a RM or even as a WRF. He's great going forward, but not played in a back 4 much though, and Nice are a very pragmatic side, and as such his defending ability in a back 4, in a proactive team who want him pushed up all the time hasn't really been tested, yet. He's very quick, so shouldn't want for pace though.
Mendy's an orthodox Left Back( who's on his way to Real Madrid), played in back 4's most of the last 2 years, who might not have Atal's goal tally (or raw pace dribbling ability) but sees about 80% more ball than him, makes nearly twice as many key passes as him.
I'd have liked both of these at Spurs. But we're definitely not getting Mendy.