I believe the statement "If SSI and SSDI is your only income then you do not even need to file a tax return" is not true in all cases. It is probably true in most cases - but not all. There are multiple situations where Social Security is your only income, but you may need or want to file a tax return. Some of those situations are for lump sum distributions, or if you receive a large amount of Social Security income, or if you had taxes withheld - you may want to file to get a refund. And yes, there are more such situations.
I suggest using the IRS Interactive Site to determine whether or not such payments are taxable
https://www.irs.gov/help/ita/are-my-social-security-or-railroad-retirement-tier-i-benefits-taxable
I further suggest using the IRS Interactive Site to determine whether or not filing a tax return if required.
https://www.irs.gov/help/ita/do-i-need-to-file-a-tax-return
For each of the above sites click the "Begin" button at the bottom of the screen.
