If Wikipedia had an article about every start up company, every product, and every software application, it would be filled with unmaintainable / uncheckable marketing spam. (To an extent it already is.) Wikipedia’s goal is to be an encyclopedia, not a software project directory.
Note that there’s a somewhat different standard between establishing notability vs. verifying particular claims in article content. Primary sources can sometimes be used for the latter. But it’s also plausible that the Signal article doesn’t establish all of its claims to the satisfaction of Wikipedia guidelines.