Slack became popular via community slacks, which the company always stated they were willing to tolerate but not interested in supporting. I don't think that qualifies as bending/breaking rules so much as repurposing. Slack still tolerates community slacks but has set precedent for setting hard limits on user counts, which has led to many community slacks migrating to Discord instead.

If anything, Slack benefitted from community slacks until they outlived their usefulness and then Discord benefitted from the influx of new users. They definitely took notice at the time and added features new users had been used to from Slack.