Honest question: What do you mean by “weight loss” and “scam”?
I haven’t used beeminder to lose weight, but I have used a similar system: a moving-average weight plot (inspired by John Walker’s book, The Hacker's Diet), plus a weight target to get down to, then a weight floor and ceiling to stay within after losing the weight. In mid 2016 I lost 9 kg using the chart. My moving-average (4 day exponentially weighted) weight has been stable at the new lower weight to within a few kg since mid 2016, stable to within one kg since late 2017, and for the last few months I’ve been trying to keep it stable to within 0.5 kg (successfully so far).
All I do, pretty much, is eat slightly less if I’m above my target weight line (which has been the same weight the whole time) – it’s not all or nothing: I eat less the more above the line I am. The “eat more when below the line” part seems to take care of itself 
I don’t calorie count because that’s a pain and the chart works as a proxy for calorie intake. My waist measurement seems to track my weight quite closely, but with higher noise. That seems true over all that time period – I don’t measure my waist daily, except for a few weeks every so often to see if it still works the same way as it used to – which it does.
It’s slightly more complicated than that, but only because I’ve learned some intuition about what is real and what isn’t. For example I know pretty closely what weight I expect to be before I weigh, so I know if my raw (not moving average) weight plummets suddenly I’m probably dehydrated. If my raw weight is up significantly several days in a row, I don’t wait for the moving average to catch up before eating less, because I know that’s likely a real change.
Before I started using this system, I was around 10 kg overweight. So not massively overweight, and one caveat here is that recently I hear theories that the biggest you’ve ever been “stays with you” via your metabolism trying to drag you back to that weight by means of increased appetite – best to view that as a good reason to start now than as a reason to give up: Mr. Walker is an example of somebody who was considerably bigger than me for whom this has worked well over a long time period – though I believe he uses a slightly more complicated system involving calorie counting.
For the record – not a terribly important detail, but: by “4 day exponentially weighted moving average”, I mean, in python/pandas: weight = raw_weight.ewm(halflife=4).mean().weight