At some point, reading kernel code is easier than speculating. The answer is actually 3. there are multiple semantics for filesystems in the VFS layer of the kernel. For example, XFS is the most prominent user of the "async" semantics; all transactions in XFS are fundamentally asynchronous. By comparison, something like ext4 uses the "standard" semantics, where actions are synchronous. These correspond to filling out different parts of the VFS structs and registering different handlers for different actions; they might as well be two distinct APIs. It is generally suspected that all filesystem semantics are broken in different ways.

Also, "hobby" is the wrong word; the lieutenant doing the yelling is paid to work on Linux and Debian. There are financial aspects to the situation; it's not solely politics or machismo, although those are both on display.