There's no proof Goku had sensed his own Super Saiyan 3 power until that time. Vegeta states Goku had a body to train and get stronger, meaning Goku is stronger than his Buu arc counterpart. To use the argument he's not including himself, you'd need to prove Goku had transformed into a Super Saiyan 3 and felt his own Ki at that point. Otherwise, Goku's statement could just mean:
The time frame for the movie, according to what all is shown, puts it, at most, a day after Goku's fight with Fat Buu, meaning that there's no real way he could have had any kind of appreciable strength gain after returning to the afterlife. As such, his strength wouldn't have changed from when he previously transformed to fight Buu.
Also, why is this situation always compared to Goku calling SVB1 the greatest Ki he's ever felt? What is even comparable about the two? How does Goku give any indication he's excluding himself when referring to SVB1?
Because Goku's
exact words when he senses Janemba (fat Janemba) is that he's never felt a ki as incredible as that, exactly matching what he said later in GT about Bebi Vegeta. Then he turns around and effortlessly takes out Janemba while a Super Saiya-jin 3. If he were including himself in his comment, then it wouldn't make sense for him to so easily beat Janemba while Ssj3.
Goku says "felt" not "sensed". You can obviously feel your own Ki, as fighters are shown comparing themselves to other people all of the time. We also know Goku and Vegeta have the memories of being Vegetto, and we know Baby has the memories of those he's possessed.
Nevertheless, as pointed out above, we still have a precedence of Goku saying the exact same thing, yet excluding himself from the mix, so it's just as possible for Goku to not be referring to anything he experienced as Vegetto when saying that.
If he thought he couldn't get any stronger why did he tell the Tuffles to give him their energy?
The same reason that Gotenks Buu referred to himself as the strongest Ma-jin, simply because, at the time, he didn't foresee himself ever needing to get stronger. He was already stronger than Goku, and therefore he had no competition, so he had achieved the "greatest of Saiya power".
The anime comic just has to mean Super Vegetto, yet two statements from the two people who have been Vegetto calling SVB1 the greatest Ki they've ever felt and the greatest of Saiyan power can mean anything? What even?
Because the entire section about that comic was the evolution of Goku, referencing all the forms and things he had achieved. Anything beyond Ssj Vegetto was something not shown, so it wouldn't make any sense to reference or suggest anything beyond that.
And I don't see any reason for this to refer to anything but ki. So base Goten is stronger than, at the very least, Majin Boo from Z. GT Trunks is comparable to Goten, and that puts him above any other version of Trunks.
Given that Goten had just finished dispatching a number of people controlled by Bebi, and then passed off his date to Mr. Satan to have him take her to safety, it could very well simply be a situation of Goten saying that Bebi is in a different league than any of the other people he just took out.