I spent a good hour searching for advanced techniques rated Ultra Hard and below but excluding chains, which would crack the puzzle.

I (of course) found the Hidden Pair [69] in r68c6. Further I found the following Y-Wing, but not the XY-Wing yielding the exact same elimination, which I needed machine assistance to spot.

The Hidden Pair [69] and the elimination r5c3<>2 did however not result in any fruitful advancement.

At this point I was pretty sure that a chain was needed to solve the puzzle. I now had JSudoku solve the puzzle and had a glance at its list of techniques used, but did not at all look at its detailing of the techniques used. It was a bit a chocking that JSudoku needed two XY-Chains plus a final XY-Wing after the two XY-Chains.

Having solved some 50 puzzles of similar difficulty by the same person who has made this one, I had a good hunch that a single XY-Chain would crack the puzzle.

For all bivalue cells I now in strict sequence tried both values to see if eliminating any of them would crack the puzzle resulting in its solving requiring Naked Singles only. All accommodating bivalue cells were marked and I started looking for a single XY-Chain cracking the puzzle. I was however not successful. Having only two bivalue cells in n4 and n5 was a major obstacle. If I remember correctly, I think I found an XYT-Chain cracking the puzzle, but discarded it as too complex.

Next I examined the bivalue cells left over from the previous step and concentrated on those whose solution would solve the puzzle requiring N/H Singles only. First out was r4c1={27}. Solving r4c1=2 -> Hidden Single 7 in r4c4 -> puzzle solves with Naked Singles. And this time I was able to find the following XY-Chain -> r4c1<>7:

Fortunately I immediately saw that r4c1<>7 -> r5c2=7<>4 and then saw that I could use a subchain of the already found XY-Chain to prove r5c7<>4. This then became my final solution. I spent some time looking for a shorter chain, also having other bivalue cells as the target cell, but without finding a shorter or better XY-Chain.

Totally I spent some 3 intense hours arriving at the published solution.