No inference was made, that's just the rules.
There are two "basic" rules for claiming an adult parent.
1. they can't have more than $5050 of taxable income (even if they don't file).
2. the child must pay more than half the parent's overall support costs.
Social security is not considered taxable income if it is their only income, but it does count as support they provide to themselves, so you have to add up the cost of living (a pro-rated share of your mortgage, food, utilities, other support) plus their personal expenses (clothes, medical, travel) and see if you provide more than half. There is a chart to help with this in publication 501.
(But beware, if mom plays Bingo or the lottery, and wins more than $5050 over the year, that is gross taxable income even though it does not have to be filed since it below $13,500.)
Then assuming the parents meet the support and gross taxable income test, you still have to be aware that the parents can't file a joint tax return (married filing jointly) (as well as things like must be a US resident, etc.)
We might assume that the parents do not file because they only have social security, but in 2021 they probably filed to get a stimulus check, and the rules might change or there might be other unusual circumstances (like tracking a solar credit carryover that might some day be used, or a one-time sale of property inherited from another relative). If we assume the parents don't file, we should still state what the rule is.
