The problem is that your mechanic trusts Shimano's claims about non-compatibility. The shifters *claim* to only work with DA mechs, but this is simply false. Shimano makes similar ridiculous claims about most of their parts.

It's likely that your mechanic lengthened your chain to allow you use of the 48/34 combination. You will never, ever use this combination on a triple unless you're bad at shifting. If there's too much slack in the chain, you can shorten it again as long as the chain is semi-capable of fitting around the big/big (with the derailleur in an ugly position); otherwise, an accidental big/big shift could mangle some drive train components.

Thought: the LHT already has really long chainstays, so 48/34 could conceivably be too long for a chain. It's still weird, though; every chain I've purchased has had loads of extra links.