Yes, as a good mod creator, you must have all packs or at least all EP and GP It is bad a modder than say I only tested with GtW, or no tested with CaD
As long as they aren't making serious enough money from making the mods (via patreon or what have you) I don't expect them to have all EPs. There's, what, close to 300 bucks worth of sims 4 stuff now?