[vedant]

High modularity refers to when a system's components can be cleanly separated and recombined.

If I were to guess, I'd say that amelius is saying that it becomes increasingly common as companies grow for them to expand into functions that were originally outside their core function. Framed this way, "selling stuff online" may have once set an effective upper bound on what Amazon does, but now they appear to be expanding into "moving stuff from place to place."

This concept is related to vertical integration.