Title is incorrect. Of these 98 days, the grid was 100% renewable-powered for only 4.84 hours per day on average (10.1 hours per day maximum). The title implies that it was renewable-powered for 24 hours per day.
The actual title of the linked pdf is:
No blackouts or cost increases due to 100 % clean, renewable electricity powering California for parts of 98 days
This article seems to be a response to accusations that variable power output of solar/wind causes blackouts. In this study, there were zero blackouts caused by 100% renewable power supply. Also, the study period was an arbitrary set of 116 days. The 98 days were not a continuous period, nor were they 98 days of a 365 day period.
(At time of this comment, the title of this post is "The California grid ran on 100% renewables with no blackouts or cost rises for a record 98 days".)