It's not, that would be `man <<< "$(curl ...)" `. `man <(curl ...)` is a slightly different version of the process substitution[1] from your comment - `man =(curl ...)`.
[1] https://zsh.sourceforge.io/Doc/Release/Expansion.html#Proces...
[1] https://zsh.sourceforge.io/Doc/Release/Expansion.html#Proces...
man <(curl ...) ends up working on Linux but for whatever reason still yields an error on macOS:
No manual entry for /dev/fd/11 curl -sL -H "Accept: text/roff" https://jamesg.blog/2024/02/28/programming-projects/ | mandoc -a