Presumably, this wouldn't be being asked unless the plan is using a graded vesting schedule. My reading of the provision is that the participant is not 100% vested. In the example, this person's Normal Retirement Age is the 5th anniversary of participation because it is the later of the two... i.e., it is later than her attaining age 65. If she has not attained the 5th anniversary prior to her termination of employment, she would not be 100% vested (though she may be partially vested under the normal vesting schedule).
I don't see the issue if you are quoting the adoption agreement for the plan at issue and the basic plan document that is to be used in connection with that adoption agreement. You follow plan terms. If you are quoting just some basic plan document that is not being used in connection with the adoption agreement for the plan at issue, I still believe that is the way that everyone I know would interpret the language. If you must have specific IRS authority for this conclusion, I don't think you will find it. (I will check back in hopes that you do.)