The traditional reading is "at the end of the mortgage" (ie, at the end of the 30 year period), not "the end of shelter-in-place". If the median age of a 30 year mortgage is 15 years, it doesn't seem unreasonable to think that borrowers will be able to pay for the payments in 15 years.
Though to be clear, there are some lenders that implementing it as "at the end of shelter-in-place", but they're in for a hard lesson.
There is also the corner case where the mortgage matures ~now, which is unfortunate, but that's not the case for most of them.
My understanding is that the traditional reading was that the mortgages would be extended for N months, so there is no balloon payment ever; just the loan lasting a bit longer than expected.
Though to be clear, there are some lenders that implementing it as "at the end of shelter-in-place", but they're in for a hard lesson.
There is also the corner case where the mortgage matures ~now, which is unfortunate, but that's not the case for most of them.