It doesn't connect directly to Braccus Rex's tomb. Larian uses grouped maps for some subsidiary areas.
If you look closely at that map - once you've found everything - you'll notice that a couple of tunnel regions are also there, running between and around the two large areas.
None of them connect to each other; it just means Larian could use one map instead of 4 or 5.
Some of the other basements and caves share maps as well. As long as you can get past the meta-gaming aspects of it without breaking your immersion, it makes a hell of a lot of game-design sense.
