I have a 2017 vintage C18, which came with 5.2 Ah batteries.
Weighed with my kitchen scale (shh....)
Bare tool, no chuck, with belt hook: 1009g / 2lb 3.6oz
With 5.2 Ah battery: 1721g / 3lb 12.7oz
With 5.2 Ah Bluetooth battery: 1726g / 3lb 12.9oz
With 4.0 Ah Bluetooth battery: 1632g / 3lb 9.5oz
With 3.1 Ah Bluetooth battery: 1440g / 3lb 2.8oz.
With 3.1 Ah Bluetooth battery and Centrotec chuck: 1488g / 3lb 4.5oz
The 3.1 Ah battery was almost completely discharged, which slightly affects its weight, but it's still quite svelte. Not a CXS, by any means, but still noticeably lighter than with the 5.2.
My guess is that the C18 weight on FUSA's website should read 2.31 lb, not 3.31 lb. And that would be with the Centrotec chuck.