ואם יעלה על הדעת שאין כל הנמצאים מלבדו מצויים הוא לבדו יהיה מצוי. ולא יבטל הוא לבטולם. שכל הנמצאים צריכין לו והוא ברוך הוא אינו צריך להם ולא לאחד מהם. לפיכך אין אמתתו כאמתת אחד מהם:
"If one were to think that nothing else existed other than G-D then He would exist and would not cease to exist like the rest of existence. Everything needs Him, but He does not need any one of them. Therefore, the truth of His [existence] is not like the truth of any other beings [existence]."
I think the reason that the Rambam points out that G-D could exist without anything else and continues to contrast the rest of creation to G-D in three ways has to do with the last line of "The truth of His existence is not like the truth of any other being[s existence]." What does the Rambam mean by pointing out that the truth of G-D's existence is different from the truth of all other being's existence? How are we meant to understand this seemingly confusing idea?
I think from the fact that the Rambam says that G-D's truth is not like other being's truth as opposed to saying G-D's truth is greater than other being's truth tells us what he is trying to say. The Rambam is not trying to say that G-D is the ultimate truth, even though that might be true, but rather he is trying to differentiate between the existence of a being that was created and a being that has always existed with no creator (aka a contingent being vs a non-contingent being). He describes the difference in the fact that if nothing else existed then G-D would still exist and G-D does not rely on any other being for His existence. Meaning, G-D's existence is as a being that exists outside of space time and without limitations, whereas all other things exist with limitations and depend on other beings in order for their existence.
The main point of the Rambam is an attempt to describe G-D. The Rambam is not trying to describe another attribute of G-D, but rather G-D's essential characteristics. Later, the Rambam talks about G-D's attributes and the actions that G-D uses in the Torah, but here he is trying to describe the core of what G-D really is all about.