My awareness of Anthony's goes back to probably about 1980. I grew up in NYC and went to many stamp shows in New York before I moved out of state. Anthony's was known at the time for coming into a bourse and buying up huge stocks of stuff from other dealers. They also were known for having very tight relationships with numerous busy estate attorneys - I think the deal was they would offer a free appraisal in exchange for first refusal on buying the philatelic holdings (I am deliberately providing an oversimplification; I am sure it was a much more detailed value proposition and was a very long time ago).
Their business model hasn't really changed since the 70s and 80s. Before the internet, it was obviously much harder to flip large stocks but if any of you remember their old Linn's ads, a lot of the stuff they sold was in either intact collections, picked collections, or large lots. So I never really thought of them as "philatelists;" they were (and are) more like very active stamp buyers and savvy "stamp sellers." They never specialized in anything that I recall.
Once on
ebay, they started breaking some materials down into smaller groupings but their core competency has always been finding/buying large estates and stocks and to flip stamps as quickly as possible.