That's not strictly true. Once you have made a booking it seems to release them. I bought a pile of points a couple of years ago and, after the stay, had no issues transferring the unused ones, plus those earned from incidental spend in the hotel, across to Avios.

Worth noting, in general, that buying Melia points on promotion is one of the best deals going. I did it for my brother over Wimbledon middle Saturday this year. Got the ME for around £160 vs £500 for cash, and via my Melia Gold from Amex Plat he was upgraded to a junior suite.