The article you linked to is a piece of gossipy speculation, and it is impossible to say for sure whether Meghan Markle was "ballistic" when she saw Catherine, Duchess of Cambridge, wearing the Festoon necklace.
However, it is possible that Meghan was upset, as the necklace is one of the Queen's heirlooms, and it is traditionally worn by the future queen. Some have speculated that Meghan may have felt that Catherine was trying to "upstage" her, or that she was trying to claim the role of future queen for herself.
It is also possible that Meghan was not upset at all, and that the article is simply making things up. After all, there is no way to know for sure what Meghan was thinking or feeling when she saw Catherine wearing the necklace.
Ultimately, the only people who know for sure how Meghan reacted are Meghan herself and Catherine. However, the article does raise some interesting questions about the relationship between the two women, and it is certainly possible that Meghan was upset.