This 19-year-old hasn't made the focus point adjustable... so you can't set a moving target at a variable distance on fire with it.
Any dish shaped thing with mirrors has a focus point - especially satellite dishes - so this isn't exactly rocket science.
Well, his *simple* prototype doesn't have that feature, but it is conceptually simple enough to implement, if technically complicated.
One, the mirrors do not have to be on a dish-shaped thing; they just all have to have the correct angle at each position as if it were on a dish shaped thing. In actuality, they can be supported on a planar platform (see Fresnel lens for a similar concept).
Two, once above point is established, each mirror can have a motorized kinematic mount behind it, to set the angle of that mirror correctly, for a desired focal point.
Once you have these, you can adjust the focus within some reasonable limit—and the range of available focal length should be long enough to set a boat on fire (although all this is probably outside the capability of ancient Greeks).