You are correct, it is not true to say Disney has been unable to attract vendors. They were in fact able to attract a grand total of 1 (one) third party (Marriott) to Flamingo Crossings - over several years - before finally giving up on the (mistaken) project. Hmm...doesn't sound like a myth to me after all.
So far, Disney has been unable to attract more than a mere handful of confirmed third party vendors to Disney Springs (several of which have opened in already existing MarketPlace or West Side locations, which presumably could have happened even without the Disney Springs expansion). The lack of hardly any tenant announcements by this point would, I have to imagine, be enough to make further prospective tenants nervous about signing agreements.