You're assuming their "core customer base" aren't spammers.

My reaction was simply validating that sending from-gmail to-gmail has high open/conversion rates, as the poster stated. However, you can only do this with a limited number of emails (500/day for Gmail or 2,000/day for G Suite). That's not to say that Google will let ANYTHING be sent between Gmail and G Suite accounts. There are lots of cases where Gmail has blocked its own users from sending to each other.

Google keeps the number pretty low (2000 daily recipients per GSuite user. Less for gmail?)

True spammers are sending spam to many orders of magnitude more recipients. They are not going to use this tool.