then what about it ?? https://www.geeksforgeeks.org/gate-gate-cs-2015-set-2-question-58/
I considered XOR Gate as 1 Gate Delay, if we consider 2 Gate Delay for XOR GATE, in that case it'll be 2n+2.
the link you posted has considered 2 Gate Delay for XOR Gate
in this pdf, they've considered NAND Gate implementation, it should help you with the idea.
thank you Aakash