They mean you can't see todo list items on your calendar, or calendar items on your kanban, because the data ends up "siloed"
they probably mean the data is less portable across multiple tools. So, you're putting data in multiple places, and creating individual relationships based on the view.
And, putting data in multiple places means extra time and money managing it. Generally.
Siobhan from Zenkit here - You nailed it, that's exactly what we mean ;)