Is the storyboard function only Captivate Draft via an IPAD?

Not aside from Captivate Draft, no.

But Captivate itself may be suitable for storyboarding - all depends on what you need your storyboards to show...

..and, of course, everyone who needs to open the CP-based storyboard would need the same licensed version of Captivate.

Alternatively, you could storyboard in PPT, then either import the final PPT slides into CP...though I wouldn't recommend that direct import approach...or you could copy/paste the content out of PPT into CP once the PPT-based storyboard is complete.