Thats not what the picture says, thats only the discsard thing
And it’s likely that the US was working on defeating more advanced ERA, since Kaktus existed in the 90s and the US likely had information about newer ERA technology (I mean the whole XB-70 → MiG-25 → F-15 thing is a pretty good example of this).
If they meant it was the only difference, then they would’ve just said difference and not “visual difference.”