Yes, that would be correct. Invites are through email. I see the text in quotes (at the bottom) when I login to the LGO portal. And in the last two years at least from what I have read through forums, not a single individual has claimed to have got an interview invite prior to the date mentioned. Also, most people got it at the same time as well, so I believe they update it in their system which releases them together via automated emails. Some people do get invites on later dates if someone backs out from their invite if they got into another school. I know at least 2 people who didn't interview last year as they had respectively accepted offers to Stanford and Harvard 1st round by that time and didn't want to just interview cz someone else could make more of that opportunity.
"Application Status for ____________
Thank you for submitting your application to the MIT LGO program.
We are currently reviewing applications and will email interview invitations soon. Interviews are by invitation only and interviews will take place during the months of January and February.
"