There was no mileage limitation on my RTW trip. If I had to guess, I'd say they're quoting you limitations on paid RTW. I found many agents who gave me incorrect info over the booking process. I'd call back at another time, and see what the next agent says.

The only basic rules they gave me was:

Move in one general direction (E-W or W-E). No backtracking, but connections are fine. So, you can connect in NRT 4 times, if necessary. I know when I checked the mileage for my trip, it came out to about 52k total, including the connections.

6 stopovers (24 hours or more), not counting start or finish. For NW, no airlines outside of Skyteam were allowed (even for US connections), which is different from what DL flyers have reported.

No open jaws.