The assumptions are stated. You may not agree with them(you obviously don't, but then no one has come close to forming a complex molecule from scratch even using intelligent methods) but they are valid for reactions showing no preference for configurations of the constituent units.
You speak of the real world. Well, in the real world, there are no "undesigned" chemical reactions outside of life that have produced the molecules being discussed. As for your speculation about the formation of the complex molecules by accidental conjugation of parts in interstellar clouds, do you realize what a vacuum is?
I'm also certain that the numbers come out the same when parts are randomly assembled. IOW, it does not matter whether one person flips a coin 100 times or 100 people flip a coin once, the probability that all of the tosses result in heads is the same.
First, given an assumption of a naturalistic origin of life, there has to be a chemical reaction that created life. Life exists, after all. Just because we haven't found that sequence doesn't mean that it isn't there.
Secondly, the formation of complex molecules (including some complex peptides) in the interstellar medium is a known quantity. Deep within the heart of Giant molecular clouds, molecular lines indicating significant quantities of organic molecules of a variety of types (including one with a significant amount of Ethanol). In 2002, the smallest amino acid, Glycine was discovered in Sagittarius B2 (18 Jul 2002 edition of New Scientist). It appears that a vacuum is not a problem in this matter.
To merge the two comments, I refer you to this paper by David Woon, "Pathways to Glycine and Other Amino Acids in Ultraviolet-irradiated Astrophysical Ices Determined via Quantum Chemical Modeling", Astrophysical Journal Letters, 2002, 571, pg L177-L180
The abstract can be read here, Abstract