What I think she means is that not every roleplayer fits every roleplayer. You will find a lot of mismatches before you find a partner or some partners that truly fits your style and will stay with you for quite some time.
(Extreme) Examples of when people might not be right for each other:
Person A writes 1 sentence posts and rarely does more while person B writes 10+ paragraph posts and rarely less. Person B might get inspirationally deprived because of the lack of content in the other persons post, while person A might be overwhelmed by the length
Person A writes in first person present tense while person B writes in third person past tense. While some don't mind, others feel the clash of styles makes it confusing.
Person A loves fantasy and rarely plays anything else, but person B wants to do a sci-fi roleplay. Person A goes out of their comfort zone but can't quite give as much to the plot because of the change of setting, thus giving all the control to person B. Person B might get bored because B has to do all the work with plot twists and moving everything forward.
There are people who doesn't mind mixing writing styles/length nor to be the leader of the roleplay. But a lot of people, especially when it comes to one x one roleplays, wants partners that does just as much as them, especially when it comes to throwing in plot twists and such. Some are lenient with the writing style though and doesn't mind if their partner does more or less. Personally I feel that it's easier to mix the styles in groups roleplays, where there are more than one pushing the story forward. If someone slack of on the story or if someone writes in a different tense than the others, it's usually not a big deal (if the GM hasn't stated that they need to make at least x paragraph's with x tense and x person perspective with good knowledge of x genre. The demands changes from game to game)
The same goes for a collaboration partner. Sometimes their style just doesn't fit your own, their ideas doesn't match what you want to do and they might not have the knowledge you need them to have. So always make sure to tell people exactly what you want from them if you're a picky person. Even when you do that you might end up with someone that doesn't fit you, because not everyone reads the rules carefully and some thinks they fit even though they in reality might not. So be ready for disappointments, but don't let them get you down. It's just to start over and keep trying.
