I'm not wrong. You can verify the truth of what I say by making a small test plan in both OmniPlan 1.7 and OmniPlan for iPad, using iTunes (App section) or email to send the plan to the Mac, then Show Package Contents in the Finder to compare. Or, you can use iTunes to add the plan file made on the Mac directly to OmniPlan on the iPad, but it won't show up in the document chooser when you launch the iPad app.
I agree that there doesn't seem to be any language clearly spelling out that OmniPlan 1.7 does not support sync. OmniPlan for iPad will sync with other copies of OmniPlan for iPad without any Mac required; perhaps they were trying to avoid confusion in that regard while creating it elsewhere. I very much doubt they are trying to mislead people into buying software they cannot use, as their refund policy costs them cold, hard cash (Apple keeps its 30% cut of the purchase price, but Omni refunds you the full amount).
Details on refunds here:
http://www.omnigroup.com/support/store/refunds/
Another approach would be to upgrade your copy of OmniPlan 1.7. If you bought OmniPlan 1.x directly from Omni, you pay half the cost of OmniPlan 2. Unfortunately, there's no upgrade pricing if you bought through the Mac App Store.