Earlier quoted context omitted.
Wouldn't it be based on send volume from your domain, and not send volume via your sending IP / smarthost?
Usually when talking about spam filtering it's based on sending IP and not domain names (domains are still important, but IP addresses are usually the first thing that is being evaluated), although admittedly Google is vague on what constitutes "5,000 mails per day".
> What is a bulk sender?
> A bulk sender is any email sender that sends close to 5,000 or more messages to personal Gmail accounts within a 24-hour period. Messages sent from the same primary domain count toward the 5,000 limit.
> Sending domains: When we calculate the 5,000-message limit, we count all messages sent from the same primary domain. For example, every day you send 2,500 messages from solarmora.com and 2,500 messages from promotions.solarmora.com to personal Gmail accounts. You’re considered a bulk sender because all 5,000 messages were sent from the same primary domain: solarmora.com. Learn about domain name basics.
> Senders who meet the above criteria at least once are permanently considered bulk senders.
IMO this is better since they have to handle all of the personal domains and small communities that send from a SMTP service like Sendgrid or Amazon SES. Relying on IPv4s to not be shared wouldn't work universally.