There's no formal rule against it but it's often done in bad taste. When people overpromote their thing in someone else's launch thread, we tend to scold them. Someone else's launch is not a great place for competitive promotion—each product or project deserves its day in the sun. On the other hand, users like to discuss alternatives and comparables, and that seems healthy.

I'd say the sweet spot is somewhere between just leaving the competitor's launch thread alone, or, if you must, then (1) mention it once and stop there; and (2) when you have any relationship with the alternative thing, disclose it.

That's the sort of thinking we apply in practice but wouldn't make a formal rule out of, partly because it's always evolving, but mainly because we don't want the list of rules to be too long. If we tried to codify all such things we'd end up with a bureaucratic list of hundreds of rules—ugh!