Except it's absolutely not. As in the source they use (ENTSO-E) absolutely has no notion of energy sources. I can also see that the other source they've listed for Croatia is offline, and according to Wayback that been the case for at least a year. In conclusion, carbon intensity per 24h might as well be pulled out of their asses because they, and I can't stress this enough, don't have that data.
Meanwhile, the website of the nuclear power plant, which is again co-owned by Croatian state-owned electricity company, tells me it's responsible for 16% of Croatia's annual electricity usage, while this website tells me it's zero. Gee, I wonder which of the two is more of an authoritative source.
Seriously, stop trying to correct someone that's local to the area and knows a bit more about the situation with your shitty global web projects that have to take a shit ton of shortcuts in order not to be empty.
There's also a graph for the origin of electricity that's available in the 24 hour view that shows different origin countries mixed in with different fuel sources.