It has been basically explicitly stated that the Well was the reason they were able to go on an imperialistic tear throughout the world, yes.
I kind of chalk up any difference in strength between the WC3 Kaldorei and the WoW ones as being due to them mostly fighting fledgling, unestablished nations (Theramore was not exactly a world power at it’s peak).
They’re still not presented as a weak race. They’re still certainly shown to be far, far more of a power than say, the Darkspear or Gnomes, who are great allies but do not really have the strongest independant armies due to their smaller territory and populace.