@NCperson
The instruction you reference is for a single, fully taxable IRA distribution. The problems that people are reporting are for multiple distributions where some are partially taxable, and are being included on 4a, while others are fully taxable and are being excluded, leading to a dollar amount on 4a that is less than 4b.
What about these IRS instructions for multiple distributions, that you are ignoring?
For IRA's: "If you (or your spouse if filing jointly) received more than one distribution, figure the taxable amount of each distribution and enter the total of the taxable amounts on line 4b. Enter the total amount of those distributions on line 4a." (page 29 of IRS 1040 instructions)
For Pensions: "If you had more than one partially taxable pension or annuity, Figure the taxable part of each separately. Enter the total of the taxable parts on Form 1040, line 4b. Enter the total pension or annuity payments received in 2018 on Form 1040, line 4a." (page 31 of IRS 1040 instructions)
