You amend your return the same way whether you mail in your return or e-filed. The FAQ is only stating about e-filed returns but the instructions are the same for even mailed in returns.
On your original return you mailed, you don't get an accepted notice but 3 to 4 weeks there should be information on the IRS website Where's My Refund, that would be the way to know they received your original return (accepted)
Amended returns must be mailed.
How to amend (change or correct) a return you already filed