@HDMI1 the term isekai doesn't relate well with chinese cultivation stuff, so
@Blackbird kinda have a point and
@PlexusDuMenton explained the differences pretty well with the lower/middle/upper tier, where in each tier there are worlds/galaxies/clusters and they can travel between planets/worlds and or lower to upper tiers as long as said character is strong enough to do it, sometimes there are even transportation services for it(depending on the story)
i think isekai does relate if the story have parallel universe and or different settings (eg. no cultivation and more about game system or technology or swords n magic kinda stuff). yu yan is from a different world yes, but the cultivation setting is the same.. and since it's chinese cultivation story, safe to assume it's a higher tier world than a parallel one