By "the email deliverability problem" I'm referring to the problem that only well-known IP addresses get to send mail that doesn't get arbitrarily thrown into spam filters. See e.g. http://liminality.xyz/the-hostile-email-landscape/ , which was front-page here a few weeks back.

If you're just talking about availability, then yes, but this was a sustained DDoS that took their servers down for hours. While the email protocol does insist that the sender should queue and try later, having no new email for hours is not really what people want out of email.

Protonmail's e-mail servers were off line for multiple days. With an outage of that length mail will start to bounce. It depends on the local configuration. But, 3 days/72 hours is pretty standard.