You don't say who (or what) identified it as phlox, but there is another possibility.

My guess would be phlox. There are many, many cultivars of Phlox paniculata. It would not surprise me to find several with brown-to-purple stems.  I'd guess it would be one whose flowers are deep red or maroon. Stem color varies not just by cultivar, but by season and by growing conditions. Look at this one:
https://www.provenwinners.com/plants/phlox/luminary-ultraviolet-tall-garden-phlox-phlox-paniculata

As for the other possibility, that would be swamp milkweed (Asclepias incarnata).  You could tell quickly if that is it by cutting a leaf.  Milkweed will bleed white milky substance.  Or wait for the bloom.  Look here:
https://www.minnesotawildflowers.info/flower/swamp-milkweed
https://www.minnesotawildflowers.info/flower/garden-phlox