You are right, however they may have added the extra joint to move or remove singular points in the motion, where a tiny angular change equates to a large movement on the work piece for example. Maybe also to get the range of motions needed in a smaller machine volume. Either way, from a pure geometric standpoint the 6th axis is redundant. In my opinion for dental applications like the one shown above it is very difficult to justify anything other than 3+1 axis milling. These 5+ axis machines are much more expensive and the products they are producing are pretty low value anyway. You do need them for implant supported bridge structures though.
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LongRat
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