Yes, that is apparently correct. From the mongodb server source code at src/mongo/bson/oid.h
:
Typical contents of the BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), a 3-byte machine id, a 2-byte process id, and a 3-byte counter. Note that the timestamp and counter fields must be stored big endian unlike the rest of BSON. This is because they are compared byte-by-byte and we want to ensure a mostly increasing order.
(emphasis mine).
There's also a lot of endian-swapping code around for the timestamp, so it seems the comment isn't outdated or anything, also the byte-ordering-thingie makes sense.