It is correct that in special relativity, where gravitational effects are not considered, any inertial frame of reference moving at a constant velocity is considered equally valid. In this context, time dilation occurs due to relative motion between two observers.
However, in the presence of gravity and when considering general relativity, the situation is different. In general relativity, acceleration and gravitational fields are deeply intertwined. According to the theory, a gravitational field can be indistinguishable from an accelerated frame of reference. This is known as the equivalence principle.
In general relativity, time dilation occurs not only due to relative motion but also in the presence of gravitational fields. The curvature of spacetime caused by mass and gravity affects the flow of time. Therefore, time dilation can occur even in the absence of relative motion between observers.
For example, when an observer is in a region with a stronger gravitational field, they will experience time passing more slowly compared to an observer in a region with a weaker gravitational field. This effect is independent of the observer's motion or acceleration and is solely due to the presence of the gravitational field.
In summary, while time dilation can be attributed to relative motion in special relativity, in general relativity, time dilation can occur in the presence of gravity and is not solely dependent on acceleration.