I think this is mostly a bit of chess history.
* Romantic chess was about fiercely and artfully attacking, often with gambits such as the King's gambit, which used to be accepted almost "as a matter of honour". For example with players like Morphy, Anderssen, ... in the middle of the 19th century.
* Classical chess, such as playing for occupation of the centre, ..., was following players such as Steinitz and Tarrasch, so this was around 1900 or so.
* Hypermodern chess is about things like not occupying the centre, but controlling it from a distance, often for example involving fianchettoed bishops. These were ideas by players such as Reti, Tartakower, and Tchigorin, maybe around the 1920s or so.
"Modern" is quite a general word, and it probably sometimes depends highly on when it is used by whom. When a book appears today and has "modern" in its title, it may refer to present-day chess maybe intending to designate chess from the last couple of years (which has been influenced by the presence of very strong chess engines), or chess from the last couple of decades, maybe since the 1970s or 1980s or so, when people began to strongly intensify the work on things like thorough opening preparation.
Really, as far as I have read, in most, if not all apertures and defenses, players strive to get domain of the center of the board. . . so did Rut Lopez, so did Alekhine, so did Capablanca. . Kasparow an so on up to today.
On tne other hand, you find papers and books about Modern Chess, even Hipermodern chess.
Could anyone explain what is it that makes a chess game classical or modern? Is there a difference in their philosophies? Are there new apertures, new defenses. . .?
Or is it a writer/editor "propaganda" for better selling?