They predate Ep2. Although CS:S launched without some of the maps that gave most of those props to Ep2, they are still definitely older. Prodigy (many White Forest props, such as control panels and bunker props, come from it)
was added to CS:S in December of 2004. De_train for CS:S (train models, handrails, utility objects in Ep2) is something like 19 years old as well.
The style in which their textures are made doesn't match the usual for HL2 and Episodes but matches all the rest of CS:S, such as keeping envmapmasks separate and using _ref suffix for them.
Some of the textures even kept the map name such as de_train_signalbox or de_train_floodlights_01.
There's no reason to think they were made for a HL2 continuation at first. None.