The video that you linked to in your question has this pinned at the top of the comments:
The DFT and IDFT are part of NumXL. NumXL is an Excel Add-in for time series analysis. Once NumXL is installed on your system, it will add scores of functions (e.g. DFT) to Excel. To download a 30-day free trial, visit our main website.
So the DFT/IDFT functions are not part of Excel, and even not part of its “Analysis ToolPack” that is a separate download for Excel from MS; but some third party tool.
Hope someone helps you getting some sort of formula to get the needed results; but actually, “I want results like in a tutorial for a closed-source third-party add-on” is a bad question; more appropriate would be “I need this math formula be implemented as Calc formula”.