"Substantially" is the operative word there. If craft beers represent 5% of the market and AB-InBev is on a path to control the other 95%, I think you could make an argument that they will have substantially lessened competition.
True. If they takeover Miller/Coors then they would be considered a monopoly. Then all they would have to do is manipulate grain and hop prices or lower their prices for a while to hurt the small breweries then jack prices when they are the only ones around.