Sorry, you are confused. The 2 rules you are thinking of are completely separate.
1. If you retire before full retirement age, and you have too much income from working, your benefit will be reduced in the following year. This rule only applies to income from working and not to other income like a pension or IRA withdrawal. This rule also does not apply in the year you retire. It had nothing to do with income taxes, only how your benefit payment is calculated.
2. At any time, your benefit may be taxable if you have other income more than a certain amount. This applies to your benefit at any time, there is no exception for the first year, and it applies to all taxable income including pensions and IRA withdrawals.
https://www.ssa.gov/benefits/retirement/planner/taxes.html
