19.1 (rev 5f248795f65bd55eb889f5803163e50479404bdd)
1.13.1 (rev 1.13.x/e219127b6e89d08f40b0796852db0636bdc322a2)
GeoServer is using a 2.3.18 version of freemarker, maybe support for that syntax has been added in a later version? The site you're referring to uses the latest version.
If you want try replacing the freemarker version in GeoServer and see if that works.
Hi Paul, did you check out if the syntax is supported by GeoServer freemarker version?