Not secret necessarily, just spamming only niche groups on telegram and discord, not publicly spamming. I believe they think this makes it less likely they get caught whilst still covering a lot of ground. For an analysis of these private telegram groups doing pumping, see the following paper. It has an index at the end listing all the private channels they used in their analysis

[0] https://www.doc.ic.ac.uk/~livshits/papers/tr/pump_and_dump_t...