I searched up this problem when I was installing my illuminas about three weeks ago. Mine didn't have the lip on that ring either to keep the bumpstop from falling below, however the answer I was given lends itself to common sense.

The bumpstop is the last line of defense for the strut so that it doesn't 'bottom out' when you hit a huge ditch or pothole that would exert a ton of force on the strut assembly. It provides a few inches of area where the piston won't compress any further and thus won't 'bottom out'.

On your original struts, that little lip really doesn't do much to hold the bumpstop in place. I know because I basically hit my bumpstop lightly with a hammer to dislodge it from the ring on the stock strut, but I hit it down instead of up, and knocked the whole bumpstop below the lip. If that little force can knock it off the ring, what makes you think it would hold it there under driving conditions like a large pothole? Its just meant to keep it at a certain spot on the shaft, having it below that ring does no damage and will have no effect on your suspension's feel or performance.

p.s. in fact I've read many posts hereof people who simply didn't install bumpstops when they switched their suspension. Smart? No. But technically you don't need it unless you're driving through any area that might possible have a large pothole or big bump of some kind.