You are right, it isn't clear to me what they are using the Leech area for.
The Leech "plan" is to take water directly from the Leech River. There are no lakes involved.
The Leech end of the tunnel is a concrete abutment located right on the Leech River, designed to take water directly from the river, and then pump it (or gravity feed it) into a creek that runs into the Diversion Reservoir (I've never seen the Sooke Lake end of the tunnel, so that's a guess - the Leech end of the tunnel however, I've been to many times).
The one time the CRD "fired the tunnel up", it just about devastated the water supply in Sooke Lake, with the solids in Sooke Lake multiplying almost instantly to almost unusable numbers.
The Leech River isn't drinking water (which any old time placer miner could have told CRD Water had they bothered to ask).
Needless to say, the CRD shut the Leech Tunnel down immediately ... and it's never been used since.
Which is not to say that the CRD isn't proceeding with "wilding" the Leech River area, shutting down roads, tearing out bridges, and otherwise turning it into a closed off buffer zone to be patrolled on just a few gated roads by CRD Watershed Security, a few dozen or so placer miners, and First Nations.