Personally I've always wondered why ALL of their 'Mechs where equipped with intro tech only, it's a bit strange.
because they knew that showing up with star league level technology or full clantech would make them stand out too much. so they retrofitted most of their mechs down to not include them. they did leave a few units with more advanced tech (like leaving CASE in, or the implication they were using MAD-1R's with CASE and FerroFib at one point), but generally it was stuff that didn't make their tech advantage blindingly obvious. they did use some clan tech covertly.. clan medical knowledge for example is heavily implied, and we know they arranged covert production of support role Omni-vehicles (though using IS tech)
remember they were meant as spies, to get a better picture of the state of the inner sphere. they clans had some intel on the IS, thanks to capturing pirates and refugees and stuff that had traveled out to the region the clan homeworlds are in, but all of that was third hand and years if not decades out of date.
they goofed on a lot of the specific models, mainly because they didn't know that some star league era designs were almost extinct, and other designs had never made production prior to the exodus, but they probably figured no one would find it odd.. not realizing that no new designs had been developed in the IS for the better part of 2 centuries. but since they had massive amounts of star league hardware to draw on from the brian caches in the homeworlds, and not a lot of the successor states hardware, they had little choice but use a lot of SLDF designs.
as it was, in the novel Wolfpack we find out they actually did show up with the full star league tech, including Warships.. and after they discovered that all of that had been lost to the IS, they refit what they could, and then cached everything else in the deep periphery. something they didn't break out until after the Clans arrived.