Google reads your Gmail. They admit it. They've never hidden it, as far as I know. If you choose to use Gmail, you've chosen to let Google's computers read, analyse, and store information about the contents of your email.
All email providers that want customers analyze and store information about the content of emails so they can provide features such as spam filtering and search.
The link, and the custom filter page that it links to, only talk about filtering by sender address and subject line, not by email content. (Although the Subject header is part of the email body.)
Google also does that if I don't use Gmail but send email to someone with a Gmail account, or send email to an address that forwards to a gmail account, etc.
