Not only not true, but somewhat meaningless.

There is no such thing as "all the iDine places" (or at least no easy way to tell what they are), because there are slight variations even between FF miles subprograms of iDine.

For one thing, United's (Mileage Plus Dining) has a feature where once you reach a threshold and you get a 25% bonus on every dine you also see "more restaurants available" (than before, not necessarily more than some other subprogram). So are "all the IDINE places" the ones you see when you're a new Mileage Plus Dining member, or the ones you see when you've reached that threshold, or what?

Second, there are definitely SOME restaurants here and there that participate in one airline's iDine subprogram but not in another's. For example, there one within walking distance of my house which is in Northwest (Wolrd Perks Dining) but not in American (AAdvantage Dining). Is that restaurants part of "all the iDine places" or not?

Finally, forgetting about the definition of "all the iDine places", Diners (Restraurants Saings Program) definitely does NOT list all of them.

I just did a search on CA, Palm Springs & Desert Area, Palm Springs/Cathedral City, and American shows 19 restaurants but DIners only shows 15. And that's in an upperclass tourist area, so Diners acceptance is understandably high.

For a more dramatic effect, look at Maine (which doesn't even have "neighborhood" breakdowns): American shows 51 iDine restaurants, Diners only shows 8!!!

But you are right on one thing:

It works both ways. Some of the places that are missing from the Diners program (but show up in others) DO take Diners, and yes, I see at least one restaurant near me that doesn't even take Amex (and certainly not Diners) yet which is listed in the Diners program. Go figure...