My understanding is that separately derived vs non-separately derived systems is all about the location of the grounded and grounding conductor bond point. If the two systems are bonded separately on the line side of the transfer switch then a switched-neutral is required to avoid having two bond points. If only one of the two is bonded prior to the switch then the transfer switch cannot switch the neutral and it is a non-separately derived system. If the bond is on the load side of the transfer switch, then switching the neutral will have no effect one way or the other and would not be necessary. This is also a non-separately derived system. I hope I am understanding this correctly.
You have it right.
From NEC 2014
100 I.
Separately Derived System. An electrical source, other
than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those
established by grounding and bonding connections.
250.30
Informational Note No. 1: An alternate ac power source,
such as an on-site generator, is not a separately derived
system if the grounded conductor is solidly interconnected
to a service-supplied system grounded conductor. An example
of such a situation is where alternate source transfer
equipment does not include a switching action in the grounded
conductor and allows it to remain solidly connected to the
service-supplied grounded conductor when the alternate
source is operational and supplying the load served.
250.30 (A)
(A) Grounded Systems. A separately derived ac system
that is grounded shall comply with 250.30(A)(1) through
(A)(8). Except as otherwise permitted in this article, a
grounded conductor shall not be connected to normally
non–current-carrying metal parts of equipment, be connected
to equipment grounding conductors, or be reconnected
to ground on the load side of the system bonding
jumper.
If the neutral of a generator is solidly connected to an electrical system,
it is not a separately derived system.