$28,300 is the standard deduction amount for checking 3 boxes in Table 7 of the 2019 version of publication 501. Right now the IRS is serving the 2019 version as the default current version, the 2020 version is still technically a draft. This is possibly the origin of the misunderstanding.
https://www.irs.gov/pub/irs-dft/p501--dft.pdf
In any case, the taxpayers here don't check 3 boxes for disabled in Table 7 because Table 7 only applies to the taxpayer and their spouse when filing jointly. A disabled child does not increase the parents' standard deduction. A disabled child gives the parents a tax credit in a different place on the form if the parents claim the child as a dependent.
