A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
As the Answer in that thread stated, there is no built-in keyboard shortcut for that purpose. However, you cannot assign custom keyboard shortcuts in Excel 2016. The other replies in that conversation apparently were from users of Excel for Windows... what they suggested merely was a roundabout way of getting into the command in the Ribbon.
If you don't mind some personal advice :-) most experienced users of Excel recommend avoiding the use of merged cells altogether. Doing so can create all sorts of limitations, problems & even workbook corruption. If you're looking to center a title across a range of columns you're far better off to use the Center Across Selection feature found in the Format> Cells> Alignment dialog in the Horizontal alignment dropdown menu. That method maintains the structural integrity of the worksheet.