That's not what 'warming up an IP' means. If Google (or any other ISP) suddenly sees an big spike in email from @example.com on an IP that @example.com hasn't

That's not what 'warming up an IP' means. If Google (or any other ISP) suddenly sees an big spike in email from @example.com on an IP that @example.com hasn't used previously, Google is much more likely to mark those emails as Spam. So senders need to 'warm up' an IP by sending a small amount of email first, usually for a few days at least. So if you're a SendGrid customer expecting to send 2,000,000 emails today, you can't just switch to a new ESP and send those same emails. Spam rates will go through the roof.