It's quite clear, if you take a bit more from the definition of obstruction. "However, a boat racing is not an obstruction to other boats unless they are required to keep clear of her or, if rule 23 applies, avoid her. A vessel under way,including a boat racing, is never a continuing obstruction."
So everything you need to keep clear of or avoid is an obstruction, but a vessel under way is not a CONTINUING obstruction, but can be an obstruction.
There is difference in the rule 19 whether the obstruction is a continuing one or not.