"Credential stuffing" as I've heard it used refers to taking username/password combos from one breached site and trying them in other sites.

So for example LinkedIn has a breach, which reveals to evildoers that user '[email protected]' uses the password 'smith1234' then they test that username and password in Amazon, Netflix, Steam and so on.

They only make one attempt per account, because they only have one leaked password per account. Hence, throttling per account isn't an option.