وَهِيَ بَسِيطَةٌ بِالنِّسْبَةِ إِلَيْهَا
It is simple relative to that comparison.
A classification as simple or compound depends on the relation being considered, including the distinction between existence itself and the existence of a qualifying relation.
“Does motion exist?” and “Is motion perpetual?” differ because of what the mind seeks to establish, not because one sentence is longer or contains more words. Simplicity in this analysis is relative to the object of inquiry.