WHO says sugar alternatives not effective for weight loss. I think this would also generally imply that they do not prevent weight gain. I think you likely just don’t gain weight regardless of sweetener; like how I and most of my family don’t gain weight regardless of what we eat. That is to say our habits and decisions don’t allow for weight gain regardless of sweetener, not that we have some genetic thing processes sugar differently or anything so unlikely.
Though yes, I also disagree with that strawman argument, diet coke being a gateway pop or artificial sweeteners being gateway sugar seems a bit rediculous.
https://www.cnn.com/2023/05/15/health/who-sweeteners-weigh-loss-guideline-wellness/index.html

Used to use Sync pro

That email seems more thought out than anything he has said to the public…