It does mean Unix timestamps. The blog post doesn’t have the full details.

You can read the RFC draft at https://datatracker.ietf.org/doc/html/draft-ietf-acme-dns-pe...

It says: CAs MUST properly parse and interpret the integer timestamp value as a UNIX timestamp (the number of seconds since 1970-01-01T00:00:00Z ignoring leap seconds) and apply the expiration correctly.