Possibly something else, yes - but generally the definition of out of true is that the wheel wobbles. If the bearing is too loosely adjusted then the wheel could also wobble, but if it always has the same back and forth pattern when turning the rim/true is the issue.

Bear in mind that simple truing (spoke adjustment) may not always correct an out of true condition. If the rim has been physically bent by impact just adjusting spokes will not correct it. If you get the rim straight but some spokes are much looser than others the rim is physically bent. I generally advise anyone who is showing little experience with wheel structure and truing to have a bike shop or experienced person tackle a bent rim.