The license is quite long.
https://www.mongodb.com/licensing/server-side-public-licenseTrying to determine what offering as a service is or implies is a bit difficult. If Discord used it to enable full text search, does they need to release their management layers too? Why or why not? How excited to defend your rationalization/decision to the court are you, if some disagreement arises?
The ask, that companies open source their management layer, concerns me because it's unclear in many circumstances what does & doesn't need to go into that box. A management layer is often a rather sprawling piece of software. Trying to disentangle & release the Elastic Search or Kibana management layer from the other management /deployment/control systems could be quite onerous.
I do think the intent is not "bad", but it's so hard & murky, there's so much peering into the crystal ball to guess whether, some day in the future, a once "open source" company that may, a decade down the road become aggressive/ligitious (not presently the case!) continues to find your particular use does or not does qualify as offering the product as a service, and does or does not expose you to a long complex obligation.