Earlier quoted context omitted.
As explained in the article, the scammers are using compromised Sendgrid domains to send the phishing emails. This means the emails are going to pass SPF/DKIM. Those domains are apparently owned by legitimate businesses which are actual Sendgrid customers. The phishers just compromised their account and API credentials
SendGrid's platform doesn't need to be the sender of these emails at all. It's just classic phishing, the emails can pass SPF, DKIM and DMARC as all of these rely on DNS resource records to be created on the RFC5321.MailFrom and/or RFC5322.From domain. Which is under control of the spammer. It's not pretending to be from sendgrid.com, if it was then these measures would help.
I can't pinpoint it exactly, but it might be a combination of the replication cycle of the attack being recursive and very short if the target is an MTA. But it may also be because the fact that sendgrid clients are sendgrid clients is public information.
Kind of how like meta companies are overrepresented in their medium, in a stock exchange banks are overrerpresented, lots of websites about building websites, lots of road ads are about placing road ads.