I don't think it's FUD. I didn't do extensive research, but I quickly googled phones with replaceable batteries and found these two from LG:

LG V20 - 5.7” display, 3200mAh battery, 174g LG V10 - 5.7” display, 3000mAh battery, 192g

And here's a phone from the same manufacturer with a soldered in battery:

LG V30 - 6” display, 3300mAh battery, 159g

That's a bigger display, a bigger battery and an overall lighter weight.

It's easy for me to believe that manufacturers are soldering in batteries to deliver thinner, lighter phones since that's what they claim, and that's what they seem to deliver. But I'm happy to look at your numbers.