From what I've read, when the Generations EP came out, it was no longer possible for a sim to travel through such narrow passages as are in that room. When WA was made, it was possible, but they never changed the tombs after it became an issue. That isn't the only place where you'll find a too narrow passage. There's also one in Tuatha's Garden in France, which you may not have tried yet, as it is a tomb that's not on a lot, with no adventure opportunity attached to it. As far as I have figured out, there are only three ways you can get your sim into that area, which you must do to get the tomb relic, because it's in that central room.
The only non-cheat way is to have you sim meditate until they can zenoport, them have them get there that way. Of course, you'll have to either zenoport them back out again, or have them use escape dust.
The second way, the one I use, is to turn on testing cheats, shift click inside the the space around the hidden room, and teleport them in, then out again when they finish.
For the third method, turn testing cheats on again, then use the BuildBuyEnabledForLot true cheat, shift-click on the floor, and choose "build on this lot" and remove one or both of the walls of the too-narrow passages in build mode.
