Except, as pointed out in the article, over time the ability to crack the underlying key and thus forge such emails becomes very practical. By disavowing the keys it prevents someone from taking a cracked key, forging an email and then "discovering it" for nefarious purposes.
Disavowing also protects against key theft or leaks. I'd guess that theft or leaks are probably more likely than cracking, now that the major providers are using long keys.
