This is non-sense. You only need to be a registered iOS developer to access the new version now because it's still in beta and apps crash all the time. The new version will be released as a free update to existing customer when it's ready (given that you have a compatible device).
Anyway, I can confirm that this works flawlessly on my iPad. Attaching multiple files worked as expected. Not just in threads but also in add-ons that use the attachment uploader correctly. This should be a great reason to have a mobile style (instead of having an app). I can't wait for that!