I note in his last paragraph that he mentions a lot about surcharged stamps. As perf12 mentioned, if he had access to a printing press is it possible that the supply of surcharged stamps could be easily met? It made me think and ask is it possible?
First, I don't think he mentions "a lot" about surcharged stamps, he talks about the 1912 Gabon issue specifically, one of which is used for the mailing above. If you look it up, it is not a particularly expensive set today because it is not very high in face value.
You are saying he took 1fr, 2fr and 5fr Navigation and Commerce stamps, likely worth a fair amount then and now, and overprinted them 10 centimes for a huge loss in Scott catalog value and real value. That makes no sense. There are no forged surcharges on forged stamps of the issue. There may be faked errors of the issue, but he was not offering anything like that.