The 5 is not redundant, and it doesn't mean you have "a four note chord."
The point of the notation is that the 7b 5 intervals from the new bass note are the same notes as the 6b 4 intervals from the previous bass note.
In other words, since the continuo is for organ (which can sustain notes indefinitely), you simply hold those two notes down, and only the bass note moves.
That is exactly the same as what the violins are playing, of course.
What is more interesting about this example is that the second chord played by the strings is actually a 6b 4 2 chord, which then becomes a complete 7 5 3 dominant 7th. But Vivaldi wanted the D played only the viola, not by the organ continuo. Otherwise the figuring would have been 6b 4 2 followed by just 7b.
A typical organ continuo registration might have made the complete 6b 4 2 chord sound muddy, especially if a cloth-eared organist voiced it with the D in the tenor, a second above the bass C!
Re the edition, you can view an alternative professionally published edition here - though I don't agree with the continuo realization: https://www.prestomusic.com/sheet-music/works/52700--vivaldi-introduzione-al-miserere-rv-638-filiae-maestae-jerusalem/browse