That link is for Piefed.world; run by the same team that run Lemmy.world

There are other piefed servers which may have different email requirements. But the most likely reason Piefed.world requires real emails to prevent bots making fake accounts and also reduce the risk of bad actors making numerous accounts to avoid bans. As it’s hard to get multiple real emails it makes it hard to make multiple anonymous accounts which is unfortunately a tactic of trolls.

You can of course create a dedicated “private” email account on an official service and use that to sign up if you’re worried about sharing your primary email account. A lot of people do this online to have a legitimate email but essentially in it’s own silo separate from other personal emails.