What does a "< <(...)" redirection mean?

No, < < and << are not the same thing.

The first is composed of the common < redirection character combined with the first character of the <(command) syntax. This is a ksh construct (also found in bash and zsh) known as process substitution that takes the output of command and provides it in a file whose name refers to the other end of the pipe command is writing to.

In other word you can think of < <(command) as < file, where file contains the output of command.

It is a convoluted way of doing the simpler:

curl -s https://raw.github.com/... | bash