An interesting site (your link). As I see it, ETH correlation decreased a few times in the past months, and even went to negative amounts, but every time it eventually returned to near 1.0 numbers.

The correlation between BTC and alts is very complicated IMHO, and it used to be different in the old times. Nowadays most of the alts seem to be closely related to BTC, and it has heavily damaged us altcoin hodlers when BTC is so bearish.