TLDR: you're basically applying the US standard to something that has been released worldwide, and US intellectual property law is known to be one of the most lax when dealing on derivatives (Feist Publications, Inc. v. Rural Telephone Service Co.). Without saying that the original broadcaster/s do not held any copyright (because, of course, there is a reasonable claim for their copyright), there are two good candidates for the Conet Project's case, both hinging on European IP laws.
The first one is the "sweat of the brow" concept, where effort (not originality, or at least not significant originality) is the determiner. Because this was released in 2001, most European jurisdictions (like Britain's "skill and labour" and Germany's Leistungsschutzrecht) still had this concept. Because the collaborators of the Conet Project did exert significant effort here (they didn't just tune, but significantly denoised and made it reasonably intelligible), it could be argued that they held a new copyright on these works. New laws now significantly tilt towards the creativity/originality concept, but this is usually not a retroactive claim.
The second claim (and the reason that I said IP laws, not specifically copyright laws) is that Europe (incl. UK and Russia) has database rights which does not exist under US law (again, Feist v. RTS). Even if the Conet Project release is ineligible for copyright in most European jurisdictions (and I doubt it due to the non-retroactivity of these laws), they can still point out that the curation of the work provided for enforcement of database rights.
There is actually a third claim (although weak), based on the first publication of a recording of a performance (phonogram rights). This also exists under US laws, although I will be sure that the first "publication" is the broadcast, especially if it was also aimed in the US. (This is the reason why "sampling" some music is considered an IP infringement.)
P.S. If you think that US IP laws are bonkers, try to navigate European IP laws (it's not even harmonized inside EU). There's even a "Copyright in Typographical Arrangement" (UK) where even assuming that the text itself is not copyright, scanning the page might put you into a lawsuit (https://cdn.nationalarchives.gov.uk/documents/copyright-typo...)
The first one is the "sweat of the brow" concept, where effort (not originality, or at least not significant originality) is the determiner. Because this was released in 2001, most European jurisdictions (like Britain's "skill and labour" and Germany's Leistungsschutzrecht) still had this concept. Because the collaborators of the Conet Project did exert significant effort here (they didn't just tune, but significantly denoised and made it reasonably intelligible), it could be argued that they held a new copyright on these works. New laws now significantly tilt towards the creativity/originality concept, but this is usually not a retroactive claim.
The second claim (and the reason that I said IP laws, not specifically copyright laws) is that Europe (incl. UK and Russia) has database rights which does not exist under US law (again, Feist v. RTS). Even if the Conet Project release is ineligible for copyright in most European jurisdictions (and I doubt it due to the non-retroactivity of these laws), they can still point out that the curation of the work provided for enforcement of database rights.
There is actually a third claim (although weak), based on the first publication of a recording of a performance (phonogram rights). This also exists under US laws, although I will be sure that the first "publication" is the broadcast, especially if it was also aimed in the US. (This is the reason why "sampling" some music is considered an IP infringement.)
P.S. If you think that US IP laws are bonkers, try to navigate European IP laws (it's not even harmonized inside EU). There's even a "Copyright in Typographical Arrangement" (UK) where even assuming that the text itself is not copyright, scanning the page might put you into a lawsuit (https://cdn.nationalarchives.gov.uk/documents/copyright-typo...)