A pipe is different from a pseudo-TTY. Pseudo-TTYs are how all graphical terminals work, but all shells use simple pipes on redirection and pipe-composition. TTYs and pseudo-TTYs are supposed to be interactive, with a user sitting on the other end with a keyboard. Plenty of tools make their output different based on if they're outputting to a tty or not, e.g. ls and others do not colourise by default if not outputting to a tty, as colour control characters are usually not what you want dumped to a text file or piped to grep. I feel this change brings curl more in-line with other terminal programs that try to be good citizens.
Psuedo-TTY's come in pairs, one end for the application to connect to, the other for whatever is consuming the output or producing input to the application. So, yes, of course they are not pipes but they are similar in that you have two end-points. So pipes and pseudo-tty's have quite a bit in common, thinking of a pseudo-tty pair as a pipe that knows how to deal with a bunch of terminal specific ioctl calls is a pretty good approximation.
Considering it a pipe is a very rough approximation, though. The only purpose of PTYs is to provide access to a TTY with all its bells and whistles (kernel provided line editing, yay!) in a world without hard terminals.
