Questions on mid-2019 Covid-19 Spread in Spain
To judge about this assay, it is important to see how they worked. They used different sets of primers to analyze for SARS-CoV-2. Namely targets IP2 und IP4 from Institute Pasteur in Paris (see reference 1) and to validate results also primers targeting the &quot;E&quot; sequence (reference 2) and two against the nucleoproteins N1 and N2 (reference 3).
Then they tested frozen samples taken between January 2018 and December 19. Of those, only the sample from March 2019 come back positive for 2 out of 5 primer pairs, none of the samples before and after tested positive, despite mentioning that samples where already positive as early as January 2020 again.
If you look at the published amplification curve you see a very late Ct value for the sample (I would eyeball it somewhere between 39 and 40), which is basically in the area of non-specific amplification. Running the PCR another 10 cycles until 50 is highly unusual in my experience.

Conclusion: I see this result as a PCR artefact and I doubt, this will be published (at least I hope it will not). The reasons for it are:

only two of five sets of primers is positive. Since these are highly specific, I would expect all five of them positive.
the Ct value is very high, around the usual endpoint of qPCR. If you let PCR run long enough, it will eventually amplify something. The also show no melting curve to prove the have a good PCR product.
it is highly unlikely that the virus appears in March 2018 in the wastewater of Barcelona and then disappears again without leaving any trace. This is clearly not the experience we have been making throughout 2020.

References:

Protocol:Real-time RT-PCRassays forthe detection
ofSARS-CoV-2InstitutPasteur, Paris
Detection of 2019 novel coronavirus (2019-nCoV) by real-time RT-PCR
CDC 2019-Novel Coronavirus (2019-nCoV) Real-Time RT-PCR Diagnostic
Panel