I doubt the clans would have been shocked or particularly angered by the use of Star League tech when they first hit the sphere. The Inner Sphere defensive forces included chassis designed and produced exclusively for the Star League (including their recent Atlas) but the clans never screamed 'where did they get that?' or 'how dare they?' when encountering one.
We tend to group a bunch of equipment together based on whether it appeared in TRO3025 designs, but in universe, that distinction would be a very arbitrary measure by 3050.
House Lords had some access to SL tech prior to The Exodus. The Exodus left factories and plenty of mechs behind for said House Lords to strangle each other over (even the big AK parked his ride on a 'roid). A significant portion of the Star League chose to stick around, keeping their mechs and techs. And Kerensky and his heirs were blind to Inner Sphere activity for centuries after that.
Even if the departing Star League had been ruthlessly thorough in purging technical records and sabotaging factories capable of producing tech the Star League considered 'exclusive', after a few centuries it would have been remarkably foolish of the Clans to assume they had destroyed all records, that the Inner Sphere hadn't reverse engineered designs that bore 'advanced' tech (and developed the ability to spam them), and that all existing ancient machines and hidden caches had been destroyed.
They should have expected at lease some tech that they might have felt exclusively protective of had proliferated, and they should have expected some advances in technology beyond the Star League's glory days, not unlike their own tech had.
The Dragoons first showed with advanced machines/tech, reflecting their expectation that Fleas, Shoguns, ER PPCs and CASE were still common enough in the Sphere. The Clans were probably surprised (and somewhat amused), if not skeptical, when Col. Wolf reported he was purging his forces of double heat sinks to avoid raising further suspicion of his unit's origins (and likely to preserve his own supplies which suddenly were not so easily replenished). Even so, this occurred very shortly after the Dragoons arrived in the Inner Sphere, and as good as Wolfnet might have become, again, it would have been foolish to assume the Dragoons had a fully comprehensive list of what combat technology existed at that time, based on even a couple of years of infiltration.
At least some of the Star League's leading-edge martial technology should have existed. If anything really surprised the Clans, it was probably the scarcity of it.
Just my 2c.