if that was true we would expect to see more lefties in the top100. if we asume that talent is distributed equally we would expect around 10-15 lefties in the top100 if there is no advantage.
Not so. (btw, two years ago there were 13 or 14 in the top 100)
Tennis requires an astonishing high level of dexterity which makes most other sports look like childs play in that respect. Because of that the variance of left handed people compared to top players who are left handed isn't skewed. There are too many formative years of practice before a player could ever get to the top 100 so the motivation for someone to be left handed, or for a left handed person to focus on tennis per se is diminished.
if there was a real advantage we would expect to see at least 20-30 lefties in the top100 which we clearly don't.
It's not merely a theory. I am left handed and cause issues for far, far better players than me simply because of my patterns of play and serve - moreso than similar level right handed players. McEnroe, Ivanisevic, Connors, Laver, Nadal all show the traits of antagonising players through awkward angles/patterns yet the reverse isn't true for them when facing a righty because they're more used to it. Even moreso lefties get the naturally favoured serve side on, on-average, more critical points (40-0, 40-30, ad-in 0-40 30-40 and ad-out) are in the ad court, including more break points. There's a reason some in the 1980s wanted lefties to have to serve from the deuce court when facing break points - because McEnroe was the king of defending break points in the ad-court.
The fact that ATP players seek out left handed hitting opponents when they're likely to face a lefty says a lot about the inherent added kookiness of facing a lefty. Similarly, nuances aside, the fact that lefties are an anomaly in the field of players makes them less comfortable to play compared to a right handed version of themselves.
Imagine for a second that Federer was left handed. The issues he has with Nadal would be reduced greatly - in fact he could look like the better player in that match-up. His serve would be better, his biggest weapon (forehand) would more often be directed at Nadal's weak point (backhand), and his own weak point would be somewhat reduced by virtue of his backhand having to mostly defend against Nadal's backhand (which is much, much weaker and lower bouncing than his forehand).