Hello Dominique
Yes you are correct.
Split horizon says, as you very well know, that you cannot learn about a route from the same interface that you advertised it out of. Because we have a loop here, R2, R3, R4, each router will always learn about 1.1.1.0/24 from an interface from which it did not learn about it. It learns about it from one interface, and informs the next router from another interface. This happens on R2, R3, R4, and then again on R2, from an interface other than the one it was sent out of. Regardless of which routing protocol is being used, the fact that we have a physical loop with redistribution can allow a route to cause such a routing malfunction and split horizon is helpless to rectify it.
I hope this has been helpful!
Laz