That is correct.
If you haven't printed/mailed yet, if you will owe some taxes, drive over to the post office tomorrow and put it in a collection bin inside to be sure it gets a postmark of April 18.
If you won't owe anything or are getting a refund, don't sweat it - you really don't even have to file for an extension.
This afternoon, I drove over to the post office to mail an extension for my business. Funniest thing I've seen - the customer counter was closed...because their internet was down! Had a sign posted - sorry, not our fault, nothing we can do about it. While I was dropping my letter off, another employee was explaining to the 5 folks sitting in the lobby for their passport appointments that they have to cancel and reschedule - can't do it without internet apparently. That was surprising - I figured they just need to review the application/photo/documents and could do data entry, if necessary, at a later time. Internet is now an integral part of life.