a bit more.
option 1: Hooke was an arch rival and also close to being a hunchback dwarf. Hence "giants" and seeing farther was an insult directly to Hooke.
option 2: Standing on shoulders of giants means that both Newton and Hooke had the same foundation from which to extrapolate their findings. Newton got up first and saw farther than Hooke. Or, basically the high faluting scientific equivalent of "Neener, Neener, Neener".