Yes, it is a custom binary format, for speed and compactness. It does however export to many formats, xml, html, text, csv etc. There's even an example JSON exporter for the scripting language. More could be added.

There's not really a standard tree file format that would allow storage that can interop with other formats. Yes, there's of course JSON and XML, but you'd end up with a file with so many custom fields it is not interoperable. And these formats are extremely inefficient compared to the current binary format.

The binary format is also able to store images inline.