The author argues that nimh batteries apart from being very inexpensive recuperate easily from long periods of being without charge and that they work perfectly with the Sierra .
The PS battery recommended by Sierra is a lead-gel and dies of old age quite rapidly. Most people replace them.
NiMH batteries should work, but be aware that they are almost as bad as the lead-gel type. If you don't recharge them often and let them go dead, they stay dead. Ask me how I know. (Or ask me how many expensive NiMH batteries are sitting in the 'recycle' box.)
Soundtraxx's board recharges the attached battery from track power and uses it to keep the sound going when the track is dead. The charge system may not be set up for an NiMH battery, which may be why they don't recommend them? If you aren't charging from track power, I suspect any 6V battery will work.
Soundtraxx made a small power regulator board, which I have used to couple the loco battery, often 22V, to the Sierra card. I think I have a spare, and I think Star Hobby, in MD, might still have one or two in stock. Give them a call.