That is not true. As it turns out, Safari didn't use that instruction [1] at the time and even when it did. 99% of those performance gain in Speedometer had nothing to do what that instruction .
I guess not really your fault. We have KOLs that put out information on their site and channels ( in this case DaringFireball and Twitter ) and they, for whatever reason never really correct themselves. ( Even when they knew they were wrong )
[1] https://twitter.com/saambarati/status/1049202132522479616?s=...