Weeb or weeaboo started off a slur and always has meant to be that way. It's not a slur against people born a certain way like race or ethnicity or sex or gender. But it fits the definition of a slur.
Or a disparagement or pejorative, if you prefer.

Merriam-Webster:
Definition of slur (Entry 1 of 4)
1a: an insulting or disparaging remark or innuendo : ASPERSION
b: a shaming or degrading effect : STAIN, STIGMA
It's never meant to make logical sense or have one true definition. It's used against people who have an interest in or appreciation for Japanese culture that another person doesn't. It's not logical, and is meant to be hostile and disparaging and unfair. A lot of people have reclaimed the word to use for themselves as a positive, taking the negative weight away from the term. But people still use it as a negative, and there's no real rhyme or reason to it. And it all depends on their own personal taste with what makes something this 'bad word'.

Final Fantasy is a Japanese video game series made by Japanese video game developers. With Japanese cultural influence and tropes. Cute races. A Samurai and Ninja class. And a location reminiscent of Feudal Japan. That is more than enough for many people who are put Japanese or even East Asian culture.

If the term had existed back when Space Invaders were first released, or the Nintendo Entertainment System. One can be certain that those fearmongering that Japanese or "Oriental" media, industry, and culture were "invading" America would have called Space Invaders or Nintendo players weeaboos. As the same mindset that caused news and media publications to fearmonger about Space Invaders really being 'Japanese Invaders' is the same mindset that lets people disparage anyone who likes something Japanese or Asian seeming that they don't as "weebs" or "weeaboos".

The reason the World of Warcraft community does this is to disparage it because it's nativism in favor of a Western developer against a Japanese developer. This same debate has been going on for a long time between Western and Japanese RPGs. A lot of gamers and publications were especially hostile towards Japanese RPGs when Final Fantasy XIII released, garnered a lot of criticism for its linearity, and Western RPGs were on the rise. And took a mainstream position previously held by Japanese Role Playing Games, Final Fantasy being most well known in the West. The RPG culture war was at its peak during that time, but never entirely went away.

Fans of very Western video games have always had a somewhat antagonist relationship with Japanese video games. And this goes back even to the days of Atari. Space Invaders still means "Japanese Invaders" to many people, the games and genres simply changed.

On a different note. While gamers often like to tout that what really matters is gameplay. They care a lot about aesthetic and general interactive experience. Final Fantasy XIV is a rival to World of Warcraft, but they are two very different looking games with very different aesthetic sensibilities. World of Warcraft is grittier and cartoonier. Fitting an aesthetic more common in Western development. There isn't a lot of cute or pretty there. While Final Fantasy XIV fits more closely with Japanese pop aesthetics. What a lot of these people are saying is that they highly value the Western aesthetics of Western video games and don't relate to East Asian pop aesthetics. And are doing it a way to be insulting and dismissive. And are being especially harsh because of a sense or rivalry. People underestimate how much gamers care about aesthetics. And gamers themselves may not be entirely honest with how much aesthetics matter in comparison to gameplay.

For some people, a game will never appeal to them if it is made by Japanese people. Or in some case, even Asian people. While for others, it is as long as the game looks Eastern or makes people with Eastern aesthetic sensibilities happy, they'll be disinterested in it, or even against it existing.