I cannot comment on RM, as I only played a handful of sessions, but D&D has evolved to
- Inform the DM of the XP budget required between character levels;
- Inform the DM of the adventuring day encounter budget;
- Inform the DM of calibrating encounter difficulty for a party;
- Inform the DM of the difficulty of skill check DCs; and
- likely a few other things I'm currently forgetting.
It's approach to roleplaying from the DM's perspective is free-form, less structured but informed by other factors than just dice.
In your example, the narrative (1) along with taking the above into account (2) and perhaps other factors (3) will steer the type of move (soft, moderate, hard) being initiated.
(1) When I'm referring to the narrative, I mean the question becomes why are they (the nomads) there? Is it a mere patrol? Were the PCs aware of patrols being done in the area beforehand? Did the nomads notice something in the distance? Were they informed of the PCs presence? Do they generally cast detect magic on a routine patrol? If not, is it not fair to first use their passive perception to see if they noticed any disturbances before expending resources? That is what I mean by narrative.
(2) When I'm referring to taking the above into account - If the nomads could be easily dispatched by the PCs, then this encounter is merely colour and not worth the RP time to run through, right? If they are indeed a challenge, what are the risks? Can one get away and report the PCs? PC death an option? Is this merely a challenge for resource (including time) attrition? if yes how hard or soft a move maybe determined depending on the adventuring day budget/milestone that needs to be satisfied.
(3) Other factors could be in-game campaign length, real-time limitations and table desires. What I mean by the latter is, if the DM is reading the table, and there is an itch amongst the players for combat, he/she may in the interest of fun introduce a scenario where combat could be a possibility should the PCs desire it.