The "glitch" is that the Primals were adjusted to guarantee the use their two hour.

Use the Garuda exploit as a clear example. People would kill Garuda before she could get her two hour off so they changed the mechanics so that after a certain phase of the fight, if you drop their HP too much they will instantly use their two hour. The same is true with Ifrit, if you keep damaging him while the nails the up, he will jump early and use his two hour without a casting time.