There is serious disagreement between serious legal scholars (Moglen, Rosen) about what kind of cooperation between components would trigger (A)GPL virality clauses. Linking counts according to one, but not the other; IIUC everyone agrees subprocesses don't count.
So, assuming said company would not want to AGPL their code (and assuming they don't distribute binaries), that choice would imply some technical decisions they would presumably not be very happy about, but overall: no.
I think the problem with the AGPL is that we don't know. The virality of the AGPL hasn't really been tested in courts of law yet, and it is easy to assume the worst, especially if you consider trying to need to explain AGPL terms to lawyers in a court case, much less a potentially non-technical juror.
It does not prevent any company from creating a Pijulhub. However, if you make modifications to Pijul or link it as a library into other code, that code must be made open-source.
(And note that GitHub doesn't even use the original git written by Linus, they wrote their own implementation, libgit2.)
> Does AGPL prevents some company creating Github like services for Pijul?
Not at all, if their service is AGPL licensed.
Of course, many companies may not want to use AGPL, and there's a whole lot of discussion on this page regarding what might/might-not be allowed to work around the AGPL. However, I didn't see anyone mention the possibility of, you know, using the AGPL, which is the entire reason the authors chose it ;)
So, assuming said company would not want to AGPL their code (and assuming they don't distribute binaries), that choice would imply some technical decisions they would presumably not be very happy about, but overall: no.