Elevation plays a part yes. Low elevation unipuerto climbs are by far the best for extreme W/kg efforts, and I do think elevation is too easily ignored when very high W/kg numbers come out. And this is arguably what makes Hindleys climb insane yesterday, as he did 6.5W/kg for almost 19 minutes at like 1700m average altitude.
Even then it's still a high value. It's not like some big GT unipuerto performances are much higher. I think Yates at Alpe di Mera, Roglic at Moncalvillo, and Roglic/Valverde at Acebo, as well as Valverde in Catalunya 2017 were all in similarish ranges.
Now while the number is raelly high it's hard to extrapolate. If you ever see the W/kg vs time graphs from CyclingGraphs or Lanterne Rouge you may note that whether a number is really high or not compared to others does not really seem to correlate with wether it was the winning or best performance of that day. Some mighty performances compared to the rest of the field can often look really weak on such graphs.
If you take Almeidas example here. Sega di Ala looks like one of his weaker climbs. Yet it's the only time he was the best of a group of WT GC riders. Meanwhile on Piancavallo, his best performance in this graph, he was hard dropped. I'm sure that even for frigging Nibali Piancavallo 2020 looks like a good performance.