Tatins are inverted tarts. If you place the fruit on top of the pastry, you are essentially making a tart. Because the caramel and apples are warm, they can potentially make the dough soggy when you place them over top and then bake the dough. You could cool them but the caramel may become too sticky and you would also be cooking the apples for a second time, so they may become mushy/overcooked. In this instance, it's better to invert it as shown in the video. Cheers!

Try to melt some butter and add a dash of calvados brandy and brush it on the apples. having tried it, its wonderful, I am sure you will love it and your apples too - even will thank you for it. :)