Because Hogwarts is Plottable
This is pretty much directly stated/implied in the books.
In the fourth book, while explaining the differences between how the Wizarding Schools hide themselves, Unplottability is used as an example of something that the non-Hogwarts schools would probably use.
“There’s traditionally been a lot of rivalry between all the magic schools. Durmstrang and Beauxbatons like to conceal their whereabouts so nobody can steal their secrets,” said Hermione matter-of-factly.
“Come off it,” said Ron, starting to laugh. “Durmstrang’s got to be about the same size as Hogwarts — how are you going to hide a great big castle?”
“But Hogwarts is hidden,” said Hermione, in surprise. “Everyone knows that . . . well, everyone who’s read Hogwarts, A History, anyway.”
“Just you, then,” said Ron. “So go on — how d’you hide a place like Hogwarts?”
“It’s bewitched,” said Hermione. “If a Muggle looks at it, all they see is a moldering old ruin with a sign over the entrance saying danger, do not enter, unsafe.”
“So Durmstrang’ll just look like a ruin to an outsider too?”
“Maybe,” said Hermione, shrugging, “or it might have Muggle-repelling charms on it, like the World Cup stadium. And to keep foreign wizards from finding it, they’ll have made it Unplottable —”
(Harry Potter and the Goblet of Fire - Chapter Eleven)
In the sixth book, it is said that the room of requirement can be made unplottable if so desired, again implying that Hogwarts usually is plottable.
“Maybe the Marauders never knew the room was there,” said Ron.
“I think it’ll be part of the magic of the room,” said Hermione. “If you need it to be Unplottable, it will be.”
(Harry Potter and the Half-Blood Prince - Chapter Twenty-One)
Furthermore, I have searched through all of Rowling's Harry Potter related writings and interviews for the string "plott", and can confirm that there are no places where it is implied that Hogwarts is unplottable.
Hogwarts being unplottable wouldn't even make much sense, seeing as it's commonly known where it is, and it's right next to a populous Wizarding city.
Also, as you mentioned in your question, Hogwarts appears on the Marauder's Map. This would by very definition mean that Hogwarts can't be unplottable.
When an area of land is made unplottable, it is impossible to chart on maps.
(Fantastic Beasts and Where To Find Them - A Breif History of Muggle
Awareness of Fantastic Beasts)
Considering all the evidence against the wiki's claim, and the lack of evidence towards it, I'm going to say that the wiki just made one of their usual errors, and that the Marauder's Map can plot Hogwarts because Hogwarts is plottable.