I can put this in simpler terms. In Islam, the acquisition of land is a priority for Muslims. And once “the banner of Islam” has flown over a land, they are never, ever, supposed to allow infidels to take it back.
They even lay claim to land in Spain they have not ruled for over 500 years.
In context, they used to rule over South Sudan. So to them it is now like Israel. They want it all back. Likely they are goaded into this by other Muslims, especially the Saudis.