In order to DoS your typical site through passphrase hashing you would need to be:

- have a ton of valid usernames/emails of accounts that need to be checked (because a typical password check will rate limit by account) - send in a massive torrent of traffic from a ton of IP addresses (because a typical password check will be rate limited by IP, even more than typical IP based rate limiting)

While this is not impossible if you had those resources it still might be easier to just DoS the site though standard pages/ endpoints by sheer traffic.