When you start a new project, you have a choice between DirectX 9 and Application runtime. DX9 runtime requires DX9, unfortunately. But DX9 is quite widespread already - it came out in 2002.
Well, Construct does require the latest DX9 iteration - the 9.0 c (bimonthly release). Well, not the latest, just August 2008 one I think?
In any case, they may need to update DX9 and they can play the game just fine. Theoretically.