The government has always kept the pre-2000 and the post-2000 separate. It has done that forever. At least since 2000 when that happened, it has been keeping those two items separate in its financial statements.
Public sector accounting standards say that when you have a funded portion of pension promise, you use the assumed rate of return on the plan assets as your discount rate. For the funded part of the plan, they use the assumed rate of return because that's what accounting standards say you use.
For the unfunded portion, you are supposed to use a discount rate based on your borrowing rate.