The true power that Ellone possessed would have allowed Ultimecia to achieve time compression...but why???

Was it the power itself or the point in time that mattered?

The events of the game suggest to me that it was the point in time. But it was Ellone's power that would enable Ultimecia to send her consciousness to that point in time.
Basically, it works like this: Ultimecia's time compression magic has to be used at two different points in history - her own time, and the distant past. Those two moments have to be sufficiently far apart in order for the magic to work. Using Odine's machine, she can send her mind a long way back, but it's not quite far enough. She has to go back even further into the past, and that's where Ellone comes in. Ultimecia sends her mind back and possesses a sorceress, and then Ellon sends her mind back to an earlier sorceress. Only then can Ultimecia's magic start to... work its magic.

Like you, I find it strange that time compression needs this tiny extra 'jump' in order to work. Ultimecia's apparently some centuries in the future, yet a few decades seem to make the difference between time compression working or failing. My 'theory' (and I use the term loosely) is that time compression magic needs to span a large period of tiime in order to work. The more time it encompasses, the more rapidly reality can be undone. Without a 'critical mass', it can't get started at all. I use the metaphor of demolition: one wrecking-ball impact weakens part of the structure; two impacts weaken two parts. If those two impacts are sufficiently far away from each other, it might be enough to compromise the entire structure.