In my personal experience - that is true. I think the reason for Thermo Lite being "less warm" is because there is a foot went and a sort of opening to get your hands out. It lets even the slightest wind in.

I did wake up somewhat sweaty when I used it them my ultralight backpacking trips. Both Therma Lite and Heatsheets offer decent alternatives to regular sleeping bags and I certainly saved several pounds in weight. As far as comfort goes - that's a different story.