there is no loophole, red bull just have a perfectly legal narrower floor.
3.12.10 In an area lying 650mm or less from the car centre line, and from 450mm forward of the rear face of the
cockpit entry template to 350mm forward of the rear wheel centre line, any intersection of any bodywork
visible from beneath the car with a lateral or longitudinal vertical plane should form one continuous line
which is visible from beneath the car.
This is what mclaren have, which is 350mm ahead of rear wheel centre line; 350 is about 20mm ahead of the rear wheel surface.
Not to be confused or combined with this:
3.12.9 In an area lying 450mm or less from the car centre line, and from 450mm forward of the rear face of the
cockpit entry template to 350mm rearward of the rear wheel centre line, any intersection of any bodywork
visible from beneath the car with a lateral or longitudinal vertical plane should form one continuous line
which is visible from beneath the car. When assessing the compliance of bodywork surfaces in this area
the aperture referred to in Article 3.12.7 need not be considered
Now redbull simply have a shorter continuous line. No loophole there,they just did a give and take with the floor area.