The title may be misleading, but IMO not for the reasons you mentioned. "90%" is based on generation right now, live. On the site from the post you can see that

The title may be misleading, but IMO not for the reasons you mentioned. "90%" is based on generation right now, live. On the site from the post you can see that for the last day (24h) renewable generation was 66%, for the last week 46%, for the last year 42%. So it's nowhere near 90% renewable in general, but it is 90% at the moment (there's sunlight and good wind). Emissions on the website from the post are lower than on the website you linked - 107 g/kWh for the week, 124 for the year - but I don't know why that is.