It isn’t that manual is bad at stop and go traffic. It is that adaptive cruise control is really great at it, and adaptive cruise control is only available on automatics.
I don't drive in stop-start traffic, so I don't know how well it would work, but my manual VW Golf MK VII has adaptive cruise control. If you need to shift up or down, you just do so and cruise control re-engages.
I'm pretty much always in 6th gear when I've got ACC enabled.
Obviously manuals have had normal cruise control for decades.