That is pure revisionism.
> It was drafted by 7 hand-picked representatives, 4 of which were basically franchists
Two out of the seven writers of the Constitution were Catalan (~1/3 of the writers for ~1/6 of the population). One of them a communist. The other one, exiled during the dictatorship for supporting Catalan nationalism.
> ratified by franchist institutions before being put to a popular vote
Before being put to a popular vote, it was ratified by the Congress that resulted from the first free elections in Spain in 40 years. The francoist party of Fraga only won 16/350 seats in that Congress; comparable to the 11 seats obtained by the Catalan party of Pujol. Half the seats had been won by centrist voters, and the other half by marxist voters.
This Congress was our "assembly of people elected democratically", that could tell the writers to go back to their drafts until the result was to their liking. Those seven writers, by the way, weren't "hand-picked": they were picked by that Congress, among their members; they were all freely-elected representatives.