If it involves cycling....of any kind, and weight loss, you're in the right place.
No worries.
Welcome to Clyde's
As far as the theory you present, the only flaw is that BMX is short, and very intense effort. For weight loss, what you want is longer term, lower intensity effort at about 65-70% of your max HR. I will say, however that a routine mixed between some endurance stuff and the BMX, you can get some serious periodization going in your routine, which is a good thing to avoid plateauing on weight loss from adaptation syndrome.