When NHTSA checked that the most fatal (but less common) crashes where off center, they started testing that way and all grades crashed (pun intended). and all makers had a hard time to improve because engine and engine bay designs made it very difficult, but that was the goal of the new test: go for the high hanging fruit for those car designs.
the tesla battery chassis doesn't have the engine, so it is easier to make changes that will beef up their car in the very same way other makers did for full fontral collision before. basically they are " cheating" the same way makers "cheated" on the full fontral collision test before.
it is very likely that other kind of crash tests, that are not even tested by anybody because internal combustion engine cars always did so well on those, are extremely fatal on teslas. but we will never know because nhsta just put teslas in the same test battery as any other car. we lack the 50+ year of continuous self improvement cycle the crash tests had with other designs so far.