Would that depend on one's definition of expensive?
The thread you posted quotes sharing miles at 3 cents per mile. That is not cheap.
I didn't read the thread in question but I've shared AA miles before and unless it has recently gone up substantially it is alot cheaper than 3c/mile
Edit: Just checked and it costs 1.2c per mile
https://buy.points.com/PointsPartne...age=EN&product=TRANSFER&CAMPAIGNCODE=MktgPgUS