You have to make a shot outside the cone. Plane having the right angle, make a sketch of a single hole and make the hole.
Then, starting from the vertical axis of the cone, make a circular repetition of the extrusion of the first hole. All you have to do is enter 10 occurrences of the hole function, all automatically spaced 360°.
PS: @ac cobra it seems to me that the angle of the holes is not the same as the angle of the slope but that doesn't change anything in your excellent tutorial. ;-) Note I prefer your method to make the first hole it's simpler ;-) I note this in a corner of my neurons. ;-)
No need for an intermediate function: we take the wizard for drilling, we click on the face of the cone (we are in 3D sketch mode), we constrain the point on a diametrical plane of the cone, we dimension it according to the need, we validate. The hole will be normal to the cone generator All you have to do is do a classic circular rehearsal.
There is also the possibility, thanks to a 3D sketch, to draw a volume (round Ø10) without merging it, and then to subtract this volume from the volume of the cone.
Your solution is very fast no need to do it Schumtz with an additional plan
I didn't know about this possibility of using the drilling assistant on a point, I still believed that it could only start from a flat sketch (so a sketch on a plan)