E-Mail is old. So old that when it was invented, "hacking" and "security" was not really something anybody thought about.
To send an email you connect to the recipients mail server and type in all the data of the mail. Including recipient, subject, mail body and importantly the address displayed in the "from" and "reply to" fields. They are all defined by the sender. The Email protocol has no way to verify if this information is correct and the sender is actually part of the aledged domain.
Today, when you send a mail, most of the time you will not connect to the recipient mail server directly, but to a "sending" mail server, which then sends the mail to the recipient. For example if you log in to gmail, you write the mail on a google Mailserver which sends it to the recipient. Or you connect to your companies exchange through outlook.
There is a modern extension to the mail protocol, which allows a domain owner to define the sending mail server which is allowed to send mails on behalf of this domain. But it is in the responsibility of the receiver to check. (Its called sender policy framework SPF)
So most likely intuit didn't do anything and the scamer just send mail without a sending mail server. And your receiving mail server did not verify the SPF correctly. Or intuit did not define an SPF. Or they did but it allows sources that do actually not belong to intuit but might be controllable by the scammers. This can happen if they want to send mails from cloud hostet systems and include them in their SPF, which may include systems by other customers of the cloud hoster.
If you want to verify mail yourself, look in the mail headers (often called: view source) and look at the "received" headers. They deta the full path the mail has taken including which system initially wrote the mail. They are ordered bottom to top, so the (chronologically) first entry is the lowest. Check if the ip adress/hostnames for the first few hops belong to intuit and if they don't, its most likely spam.
TLDR: what is necessary to send mails from somebody else's domain? Nothing. You can just do that. Mail is insecure by design and should be abolished.
You didn't read the article did you? Its not about the inclusion of a character, but about how a specific scene with that character is handled. The author claims it is completely jarring, doesn't fit into the games setting and doesn't even use the games existing lore for transgender people but instead uses modern terminology.
I found the article to very informative and not at all "gamergatey".
Its points are: