Hello,
Telemac is not an acronym, it comes from two previous computer programmes, one called Ulysse, solving 2D Navier-Stokes equations, and the other called Mach1, solving compressible isentropic Euler equation (which are formally like Saint-Venant equations), so Telemac reminds of Ulysse, as Telemaque (name in French) was the son of Ulysses, and of Mach1. the ending "mac" was probably coined because it was the time of the first Apple MacIntosh that we had at that time. Well sorry, it is difficult to explain this in a paper...
I forgot for Sisyphe, it first started as a national program, before joining Telemac, and the name was probably coined by one of the first developers, Bernard Latteux, an expert in acronyms, who is retired now.
Regards,
JMH