Your imgur link isn't working. But looking at those MuseScore 4 shots and comparing them to what I see from the Steinberg website, what I see is mostly superficial aesthetics which are on trend all over—not just notation apps.
Certainly no more blatant than the continual tit-for-tat "ripping off" between iOS and Android.
I tried your imgur link again and it worked this time. What I’m seeing here is a couple of similar arbitrary locations for UI elements, similar use of colour, similar flat design popularised by Microsoft and many elements that are common among most if not all modern sequencers.
I get the frustration that arises from a competitor having a similar aesthetic but I’ve used software long enough to know that what makes software great is the details, the nuances, and everything else which distinguishes a program from a Winamp skin.
