I guess they can check logs and find how many times this has been abused already? Can we trust them to release full transparent report?
Generally browser vendors take a pretty dim view of CA's not being transparent when bad things happen. Given the seriousness of this issue,i suspect being aggressively transparent is their only hope of saving their business.
Depending on what was logged during the validation, it might be tricky to determine if it was abuse or not. If the DNS content wasn't logged, they could pull a live record and report if the current record would support validation or not.
My guess is that use of this method should be low... If you're updating DNS to add a TXT record, you might be more likely to add a direct verification value rather than an email. But that's speculative; I'm not a CA, I've just been a customer of several... IIRC, I've validated domain control by controlling postmaster@ (or the whois address when that was public) or adding direct TXT verification records or ACME http validations.