I don't think people should jump so quickly on this theory... Regarding the little information given within the series; subtle information such as you posted is sometimes the only thing we can base our theories on.
Now Ganon(dorf) is the only person to truly hold the term "Demon King", Ghirahim self proclaims him as the "Lord". However he hasn't earned the title so to say. It's also important to note that Ganon(dorf) only earns this title when claiming the Triforce of Power - thus he turns into his all powerful beast form.
However, what exactly does the term "Demon King" mean? Well it's the ruler of Demons of course; however, Ganon's usual army is made up of the incarnations and diverse forms of "Blins" e.g Bokoblins, Moblins, Miniblins and Bulblins. Now these races aren't demons, let's look at the definition of a demon:
Demon--> An evil spirit or devil, esp. one thought to possess a person or act as a tormentor in hell.
Now i like this definition more than the others given; now we also know of a so called "Demon Army" - this was Demise's army showcased by a specific number of figures seen in the prologue to Skyward Sword. Based off "loose" theory, Ghirahim bears resemblance to one of these figures. Disregarding this loose evidence though and we still know that Ghirahim served Demise and fits the definition of a demon down to a tee.
So the title "Demon King" bears a lot of significance in my eyes, now one can easily notice the comparisons between Malladus and Ganondorf, especially while in Beast form. Although the giant floating head--as someone earlier mentioned--also bears resemblance to the floating head figure we saw in Twilight Princess . This form of both beings seem to be more of a spiritual adaptation rather than a true physical form, Malladus was probably weakened, that is why he needed to posses someone in order to return to his true form.
I would say that Malladus is in fact another incarnation of the hatred of Demise simply because the arguments before are much greater than the arguments against. Ganondorf was killed in The Wind Waker, so this could warrant a new incarnation as Demise explains himself his hate never perishes. However, one could argue this with the new incarnation of Ganondorf in Four Swords Adventures. To counter this argument though, the Gerudo Race were still very much evident on the Child Timeline, yet on the Adult Timeline they seem to have diminished, so this would mean that a new Ganondorf wouldn't have been able to be born. All in all, a very plausible theory and I'm sure many people have made such connections before but I guess it will never be explained where exactly Malladus originated from.