Your book is correct to a certain extent, although it's a little voracious in its claims. Sinularia make great tank mates for other soft corals, and will even do well with most other significantly hardy LPS. They do release alleopathic chemicals into the tank which will inhibit the growth of sensetive LPS, clams, and SPS. These can be controlled with carbon filtration though if you do wish to keep a mixed reef. Stronger LPS like brain corals, bubble corals, and Euphyllia genus corals do just fine with softies in the same tank. Furthermore if a sinularia is allowed to touch the skeletons of other hard corals, its rough skin will abrade them to death. This isn't a problem with brains, bubbles, or euphyllia though since their strong stinger tentacles keep the softies at bay, but SPS do not have such defense mechanisms and the physical contact of a sinularia would kill them.
Sinularia do play very nice with other soft corals. Sacrophytons, devils hand, kenya tree, xenia, palythoa, zoonathids, pretty much any other soft coral will be just fine with them.
HTH