He meant can be revive more of the same mega-beast type (or mega beasts of different types).
What the manual means you can use a mega beast soul for...
A) Training a soldier to 15 skill level in 1 skill. This means you CANNOT revive the megabeast.
or
B) Revive the megabeast, but you cannot train a soldier to 15 with its soul (obviously).
To my grasping, you can revive as many as you want!... but they are friendly, and building-destroyer tagged still (last I tested, eons ago) and they will go around breaking every building, door, and hatch in your fort while you can do jack squat to stop him. Aka, they're fairly useless. You can weaponize them somewhat, but compared to one of the various turrets they are just not worth the hassle imo.