I don't see any reason to believe that he's lying. 300+ guys can be pretty strong. I'm not quite 300, but I'm still a big boy (6'00" and 293 as of Friday) who moves a decent amount of iron for a 30-something gym rat with less than two years under the bar. I still get an occasional piece of unsolicited advice from lean and weak men who make too many assumptions about me, my goals and the path I've traveled.
The OP did not go to great lengths to explain himself, but I definitely understand his sentiment.
OP, just keep working hard and prove your doubters wrong. The weight comes off when you do what you need to do to lose, which may or may not have overlap with doing what you need to do to get stronger.
Just keep lifting, keep improving and don't ever quit.