Looking at claimed numbers for the S21E, the 519A appears to have a 6A linear driver while the B35A has a 2.2A output boost driver. Djozz shows the 519A making 1370lm at 6A while this post has this B35A making a hair under 1500 at 2.2A.
Convoy, however is probably not using as high a flux bin as this; one product image shows their 4500K B35A being E800, so there’s probably no significant difference in maximum output compared to the 519A. The B35A is doing it with about 2.2A * 6.0V = 13.2W though, and the 519A is using 6.0A * 3.3V = 19.8W.
That’s what the LEDs are actually using anyway. On a full battery, that linear driver will pull 6.0A * 4.2V = 25.2W, making 5.4W of waste heat.