As the biscuit is oval in shape (I don’t know the more accurate description of the shape if there is one), I think you’d want the slot to hold the biscuit in place so the middle/biggest part of it enters both pieces without the chance of swiveling to get the best from it, hope that make sense. Plus when you’re adding glue, you would have to add glue to fill the slot and with MDF you don’t want it expanding. I am not an expert btw.
Part of what holds the joint together is the biscuit swelling from the moisture in the glue to fill all the voids and lock itself in. Not sure if it’d be as structurally sound if there was a large area for the biscuit to swell into in the corners. Is it a big enough difference to matter? Probably not.
I always thought of it swelling in the direction of its thickness, not its length or width.
The void isn’t helping anything, for sure. I just wonder how much you could get away with if you just made it a constant depth.
But I don’t have the time to test it either, so 
I will say I have seen makers make a big slot with a circular saw for a row of biscuits. But it’s hard to tell from youtube if the piece is actually strong or not.
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I’ve seen them do that too.
In the past, I’ve cut a 1/4" slot in two boards and then used a thin long piece of 1/4" plywood as a biscuit.
Like you, I’m not really sure how much of a difference it would really make.
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When you consider that the whole reason biscuits are shaped the way they are is so rhat you can use basically a circular saw to cut the slots for them, if you wanted to cut slots… why use a biscuit? It eould be more than simple enough to cut any piece of wood to fit a simple slot.
Biscuits are generally compressed a little to give the join more firmness, but thats noy where the strength comes from. That is surface area of the 2 pirces being joined and glued. The biscuit isnt there for strength (Seriously, those things are almost brittle) it’s there to provide a solid alignment in 2 dimensions, plus rotation. If you want strength, you go with a dowel or a pin, which can be made from strong material. (Also easy and inexpensive to CNC)
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