This is the shape of the dome on this piston according to the website you linked to:
There may be a couple of issues with this piston.
It's not a 'true' flat top - it has a dome.
You could machine it dead flat - but then you'd have to worry about the crown thickness (it might still be ok, ideally you'd want to purchase 1 piston and cut through it with a band saw to see if it will still have enough thickness after machining to be safe).
If you wanted 'really' high compression, you could machine the dome to match the chamber shape, and deck the block etc, so that the piston 'dome' section sat above the engine block deck height.
If it does have to be machined dead flat, well the crown height on the listing is potentially measured to the top of the dome, not the outer edge (I don't know for sure, it might not be). If that is the case, then once it is machined, it'd end up sitting a mm or more down in the bore.
Of course you could machine the block to get zero deck height, or offset grind the crank and get a bit more stroke, and hence raise the piston a little at TDC (you'd probably only get it about 1mm higher absolute max, and even that might require the crank journals be welded up and re-ground to have enough meat for that much increase in stroke.
Alternatively you could look at slightly longer rods. (not cheap of course).