Yes of course. That's a central element of its rail policy since 1996. It is also extremely difficult and costy to implement.

Does DB use similar leverage to limit access by SNCF

SNCF had no ambitions until recently to run competitive services across Germany, so it's difficult to tell. There is (theoretically) open competition on long-distance services in the EU since 1 January 2010, and SNCF announced plans to run trains from Strasbourg to Frankfurt and then branching to Hamburg and Berlin from the end of this year. However, realising that the approval of locomotives and coaches would take longer, they calculated that buying the timetable slots for 4 years but using it for only 2½ years won't bring profit, and dropped the plans.

Note that, on the basis of cooperation, there are TGVs running in Germany: SNCF's own run from Strasbourg to Karlsruhe, Stuttgart and Munich (and, when ICEs have problems, also on the other route via Saarbrücken to Frankfurt); and those of international consortium Thalys (in which both SNCF and DB have a stake) run from Brussels to Cologne.

the claim that the Chunnel has brought few if any economic benefits to the regions on both sides of the Chunnel

Other than that it's dated? It's true though that the failure to establish a cross-Channel regional service (proposed in recent years) limits any regional effect.

or the economy of the UK?

The Chunnel has been a financial disaster, no question about it. Only the recent debt restructuring brought it into the black. But I blame the messy private financing and the delay in the British connecting line, both crimes of Maggie Thatcher.

_{*Lunatic*, n.
One whose delusions are out of fashion.}