The movie glossed over some details. In the book, Soylent was a variety of different things, with a soya and lentil base (hence the name), and only Soylent Green specifically was the problematic one, it being the newest release. (I haven't read it myself, just summarizing what was discussed in the last thread.)
I can't find the link where I read this now, but apparently Soylent was largely composed of soya and lentils in the book version. See also the published list of ingredients to assuage your fears.
If not, isn't soylent just about the worst possible name for a real food product (with unknown/nonspecific ingredients), because of the movie?