Endo-steel requires orbital factories, unlike ferro-fibrous, XL engines and double heat sinks. Endo-steel repairs are more problematic because we're dealing with internal structure. And since it is rarer than standard structure due to its manufacturing requirements, endo-steel is a logistical problem.
One of the Wolves' better points has always been their good logistics. Consider their Inner Sphere conquest and Tukayyid campaign. (This is also presumably the reason why they thought they could keep Homeworlds-Inner Sphere supply lines working rather than starting manufacturing in local factories, naturally this plan put all eggs in one basket...) But even with good logistics, there are limits. So, they make a superb 'Mech for their own purposes but also make a bit different 'Mech for their own purposes and for trading.
So, yeah, in a sense the Gargoyle feels like an "export version". Though this feels as if they were selling F/A-18 (medium-weight multirole plane) to customers who really want/need F-16 (light-weight air superiority fighter).
The Gargoyle does supplement the Timber Wolf in some ways: Namely carrying battle armor and if we look at the Gargoyle Prime, anti-air/anti-vehicle support (hence my F/A-18 comparison); whereas the Timber Wolf is not a great BA taxi without sacrificing some attack capability, and Timber Wolf configurations are often seemingly dedicated for killing 'Mechs.
I'm going to assume the Wolves figured that other Clans would be interested in the Gargoyle because it descents from the Woodsman, which must've been highly regarded given how good it is. Figure bidding tactics might be in play here too: Anytime the Timber Wolf manufacturing rights are sought, they'd be defended with elite forces, while the Gargoyle wouldn't be controlled as tightly, further deflecting attention from Timber Wolves and keeping them to the Wolves themselves.
EDIT The major flaw here is that you could squeeze what the Gargoyle does to a smaller and cheaper chassis without losing anything really...