There's a typo in the above; I quoted 3b), not 2b).
But 3a) is even more squarely on point:
" a) Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,"
That's what closes the source-printed-on-paper loophole. Not the "preferred form of the work for making modifications to it."
Put differently, if you're right about the "preferred form" wording being the cause of the elimination of the paper loophole, then the "medium customarily used for software interchange" language becomes redundant and therefore meaningless.
Ask an attorney (I'm not one), and they will tell you that there are standard rules of construction that courts use when interpreting legal documents that strive *not* to create redundancies or rob language of meaning.