They don't claim to have invented CDC, or FastCDC, they just made and are sharing a useful implementation of it.
And if that Tarsnap presentation is from 2013, and FastCDC was published in 2016 [1] according to Wikipedia [2], then presumably Tarsnap didn't invent FastCDC either.
Another well-cited predecessor is "A low-bandwidth network file system." (https://dl.acm.org/doi/abs/10.1145/502034.502052), which was published in 2001. It uses Rabin fingerprinting to define chunk boundaries.
And if that Tarsnap presentation is from 2013, and FastCDC was published in 2016 [1] according to Wikipedia [2], then presumably Tarsnap didn't invent FastCDC either.
[1] https://www.usenix.org/system/files/conference/atc16/atc16-p...
[2] https://en.wikipedia.org/wiki/Rolling_hash#Gear_fingerprint_...