In the anime and english translated manga, it is called Demon Lord, rather than Magic King, because both are pronounced "Maou" in Japanese.
I have no idea how this statement, and the logic behind it, has existed on this wiki for almost half a decade, and spread across nearly all the articles referencing the concept. It makes me wonder if the person who decided to perpetuate this nonsense even knew basic Japanese.
The manga uses the the term 魔王, which, literally translated, means "Demon King": The 魔 Kanji (pronounced Ma in this case) broadly represents the concepts of of demon, devil, witch, evil spirit, evil influence, dark magic, etc, essentially embodying the negative/bad (although not always supernatural), and the 王 Kanji (Ou) essentially means "King".
This is the exact same term so commonly used in fantasy manga, light novels anime and games to refer to the 'Big Bad', leader of the forces of evil, etc, who sometimes is an actual demon, other times just a crazy-powerful bad guy. This is why it is also sometimes localized as "Overlord" instead, as a "Demon King" who isn't, you know, actually a demon, doesn't make much sense to Westerners. It can also refer to the literal Christian Devil (The guy who runs hell). The most common localization these days, to the point that it has effectively become translation convention, is "Demon Lord". It does not, and never has or ever will, translate to "Magic King". Nor would any form of translation or localization ever render "Magic King" into "Maou" in Japanese.
The Western concept of magic is relatively new to Japan, on a historical scale; new concepts required new words, and Western magic was heavily represented by traditional "Black" magic, such as spells to cause harm and destruction (think: Fireball), creepy magic circles, monster/demon summoning, and the possibility of the magic corrupting the wielder. As such, it got the 魔 (Ma), and it is now intrinsically linked to the terminology; you have:
Terms for Magic:
魔道 (Madou) - literally: The Way of the Demon/The Path of Evil, etc.
魔法 (Mahou) - literally: Law of the Demon/Law of Supernatural Evil, etc. Note that 'Law' here is used in the sense of the hidden underlying rules and principles governing phenomena, rather than the legal sense.
魔術 (Majutsu) - literally: The Art of the Demon/Witchcraft/Dark Arts, etc.
Terms for Magic-Users:
魔道士 (Madoushi) - Practitioner of Madou
魔法使い (Mahoutsukai) - literally: User of Mahou
魔術士 (Majutsushi) - Practitioner of Majutsu
The different terms are often used in different scenarios, referring to slightly different concepts (Such as a "Mahoutsukai" being contrasted to a "Majutsushi", with the former having a natural affinity for, and innate control of, magic, whereas the latter has learned/developed techniques and tools to control magic without any inborn talent, as seen in 'Mahoutsukai no Yome'). The point here is that, when referring to the concept of Magic, 魔 (Ma) isn't used by itself. 魔物 (Mamono) refers to a supernatural monster, creature or demon. 魔神 (Majin) refers to a devil/demon.
For terms referring to magic, you need something like the terms above, i.e. 魔法少女 (Mahou Shoujo), the quintessential Magical Girl.
If you want something like "Magic King", you need to have a word for Magic in the term. For example, in 'Overlord', you have the protagonist taking on the title of 魔道王 (Madou Ou), localized in the West as "Sorcerer King". In 'Black Clover', the protagonist dreams of becoming the 魔法帝 (Mahoutei), literally "Magic Emperor", localized as "Wizard King". Either of those you could conceivably get away with translating as "Magic King".
But translating "Maou" as "Magic King" is just plain incorrect, one step away from translating it as "Ice-cream Sandwich". Even Google Translate wouldn't come up with that.