No. I can explain it more easily with this comparison:
Imagine the file of a video in mp4 as a timeline. You edit stuff in a video editor to create a video, you can create a process of things A, B & C in the first 3 seconds, save the video, create a process of things D & E in the next 2 seconds, load back to where only things A, B & C existed, and then create other different things in the next 2 seconds. But, which is bigger?; Any file of a save or the whole video? The video will always be bigger. Why? The video is the whole information organized together, the saves are how to organize that info, emphasis on how to, saves aren't that information. In fact, all saves of a video need that video to work, without it or part of it, what are they going to organize?
SAVEs were never said nor implied in Undertale to be timelines, and this is unlikely to be the case given what a save is and even what its inspirations could have been (saving honestly in a game, saving an image or video in a program, or saving and loading dishonestly in a game via emulator, the saves are all the same, just organizing), yes after a load in any "product" it may look that, as the possibilities can be others, the "product" is another all together, but the save never had the "product" stored inside itself (that's insane), only some info on how to organize it, the user made that "other product".
If in Undertale P.Flowey could SAVE a point A in time, make a domino effect of things 1, 2, 3, 4 & 5, all numbers needing the previous to exist, then Flowey SAVEs a point B in time, then LOADs back at point A, then makes wrong the domino effect of things, having 1, 44, 0 & -20, and then LOADed back at point B, then congratulations, that future should not exist and what he does is way more than going back in time. But it doesn't prove SAVEs are timelines, a video editor can do the same. P.Flowey would just have Causality Manip, which structures events in the past, present and isn't a Low 2-C feat every time someone uses it. Going back to the video=timeline comparison, a video is all the time in a timeline, all the space, and all the causality from beginning to end, and a save is just all the causality from beginning to end, with no time and space. Now, in Undertale, I'm pretty it's just Time Manip to go back in time, but if it isn't then the SAVE is not causality from the beginning to end of a timeline, it starts when the characters gain their powers and finishes when the characters are done creating their SAVEs, the rest of the future of their to create and their past before having their powers is never touched.
