The UB in this case is "you may get another value in the sandboxed memory region if you dereference an invalid pointer, rather than a guaranteed trap". You can still have UB even in a sandbox.
Seems like they got overly attached to the guaranteed trapping they got on 32-bit and wanted to keep it even though it's totally not worth the cost of bounds checking every pointer access. Save the trapping for debug mode only.
Ah, so you meant UB = unspecified behavior, not UB = undefined behavior.
Maybe. Bugs that come from spooky behavior at a distance are notoriously hard to debug, especially in production, and it's worthwile to pay for it to avoid that.
[1]: http://catb.org/jargon/html/N/nasal-demons.html