Honestly, it's not so much of a trick, but that the form is a dark pattern.

IMHO, they probably really, really want to require email addresses to register, but are trying to avoid the backlash from power users (aka their content source) that would happen if there wasn't a workaround. Every time someone gets ticked off about this, there's probably a comment within 5 minutes with instructions, and all outrage energy drains away.