I found this that suggests the following:
I went to look for this setting but cannot find it:
Is there an alternative way to send emails in the background in Gmail?
It looks like that labs entry was either removed or incorporated into gmail.
This is from the link you provided in your question:
Note that background sending is currently the default (and only) option in Gmail. You need not enable or do anything to have Gmail send in the background.
also note the blog entry was updated on September 16 2016.