Couldn't use mounts during P4?

[Matsume]

Also take note that until you use the chococo whistle you can't actually ride any other mount.
That's because you only get access to your mounts after the L21 quest wherein you join a Grand Company and can then buy a chocobo issuance, which gives you the whistle.
Thus, without having used the whistle first, no mounts.

Not true. I was able to ride my mounts and I have yet to join a Grand Company.