Or they could use email but via someone other than Mailchimp, such as ActiveCampaign, MailerLite, Sendinblue, GetResponse, Moosend, EngageBay, Omnisend, Zoho, Mailjet, ConvertKit, Benchmark, AWeber, HubSpot, Constant Contact, DotDigital, Campaign Monitor, Sender, SendPulse, Klaviyo, Jilt, Autopilot, Drip, Keap, and probably many others.

Email is not like, say, Facebook where to reach someone on Facebook you need to send via Facebook. How you send email is largely irrelevant as long as you pick a provider that doesn't send so much spam that their outgoing mail gets blocked.