Articles – ‘A Modern World’ vs ‘The Modern World’

The definite article locates the noun it defines within the discourse context, so any question about use the definite article must take context into account. Here are the first two sentences of Lovecraft's story:

From even the greatest of horrors irony is seldom absent. Sometimes it enters directly into the composition of the events, while sometimes it relates only to their fortuitous position among persons and places.

It is clear that the events are those which are previously referred to as the greatest of horrors.

You can't really say he is world's greatest chess player. Nor can you say what in world are you talking about. The sentences are just not grammatical. You absolutely must have a definite article placed in front of the word world. That's because it's usually a requirement that you use some sort of article in front of countable nouns in English and if the thing that you're talking about has already been mentioned, or is common knowledge, or is about to be defined, the article to use is the.

However, it's a totally different story when it comes to the way we use the word Earth. You can leave out the the from the Earth (notice that this word usually begins with a capital letter). But in that case, what you're doing is that you're referring to the planet we all live on by its name. All planets in our solar system, as you probably know, have been given names: Mars, Pluto, Venus, Jupiter, Neptune etc. Earth is just one of them. Earth, in this respect, is a unique word as it can be used both with a definite article (the Earth) and without one (Earth).

As per the article that you've been reading, the fact that there is no definite article in front of the word world is simply a grammatical mistake on the part of the people who wrote it. Simple as that.