Having been a long-time user of DrawPlus, I just assumed that AD would open DrawPlus files, now I do understand what "assume" really means.
AD "shares no code with" Adobe Illustrator, but still manages to import its files.
I'm sure the 80-20 rule applies, I would guess that most basic objects could be imported, and warnings flagged for objects not understood.
At the very least, there should be something on the advertising "Features" to warn people that DrawPlus files cannot be imported, to avoid others making the same assumption.