The bug

You can knock back armor stands with a sword. See bug.mp4.
This could be maybe considered as works as intended for regular armor stands, but definitely not so for armor stands whose Marker-tag is set to true; those shouldn't be affected at all by sweep attacks or anything at all, except commands, as they're used by mapmakers who partially also still have to rely on Marker-true-Armor stands.

How to reproduce

1. Give yourself an armor stand with Marker set to true, e.g. with:

   /give @p minecraft:armor_stand{EntityTag:{CustomNameVisible:1b,Marker:1b,CustomName:"{\"text\":\"Marker 1\"}"}} 1
   

2. Place a couple of those armor stands close to each other.
3. Spawn an entity with a spawn egg (Pigmen work well) right in front of or also inbetween those Marker-true-Armor stands, notice the armor stands get knock-backed as well.

With the above test setup, this doesn't seem to be reproducible while you're flying (hovering) or swimming.