Citation needed. The closest thing she says to this is "Well, Mango's going out with Jackie now, so he'll probably have less time to hang out with me" to Oskar - this is how she fears the summer will be, not a statement of how things have actually been so far. It also comes during the time when things have been awkward between the two of them after Face the Music, when they somewhat distanced themselves from each other. This wasn't the case throughout the majority of the season.
Also, in one of the live chats Marco was asked if he and Jackie were officially a couple, and he stammered answering it, saying "define official", but also he said that he and Jackie hadn't been on a date again since Bon Bon. While it's possible that Marco and Jackie hung out again off-screen at some point after the live chat but before Just Friends, if nothing else it shows they have less time to do so, and generally just shows that they are not really that close, not enough to be really considered a couple yet.