How to achieve fall-off-the-bone lamb with browned bits in oven?
That temperature is fine, but is never, ever going to produce “fall off the bone” in one hour. That requires hydrolysis of collagen into gelatin, which takes significantly longer.  It’s possible the chef was describing how he finished the lamb, after a longer (and possibly wetter) cook at a lower temperature.
Don’t worry too much about losing liquid. That is absolutely going to happen with long, slow cooking, even if you’re boiling the meat. The “moistness” of long-roasted meat comes from rendered gelatin, not from hoarding the juices. With very long dry cooking you do run some risk of drying out (this will manifest as a thick dry 'bark' on the meat, not as dryness throughout), but just tenting the meat with foil should avoid that.
Try three hours at 180, covering with foil until the last half hour. This will probably produce more fall-off-the-bone than you'd like, but it's a good starting point.