Seems like the author never truly worked with these engines? Also, I never had problem with unit testing on Unity in pure C#. Even when it comes to complicated scenario there is always a way to separate your GameObjects from game logic.
I think this comment and its parent have misunderstood what ECS is.
I have spent way too much time in Unity. (I was a Unity developer for pay for several years.) I suppose I didn't do a good job explaining the differences between what Unity does and an ECS approach in my post, but if you'd like to get a feel for what an ECS you could try out DOTS. (Although DOTS is kindn of unpleasant to use compared to Bevy.)