There's no outward indication of the authors. It looks official enough, but my experience is that government/foundation types like to splash their brand all over their works, and there's no branding.
I was asking myself the same thing. Is the "CIRCL - Computer Incident Response Center Luxembourg" a partner organisation to CERT? I don't follow this very closely, but it felt like there was some kind of global network. I'd be curious if there was a continuity plan of some sort. There is also https://cert.europa.eu/ which might be used interchangeably.
CIRCL, the supposed operator behind gcve.eu [1], "is the CERT for the private sector, communes and non-governmental entities in Luxembourg" [2].
[1] https://gcve.eu/contact/
[2] https://en.wikipedia.org/wiki/Computer_emergency_response_te...