A non-backbone area should not generate summary LSAs. What number 1 is saying is that any summary LSA coming form a non-backbone area are ignored. An ABR will have at least one interface in Area 0 (backbone), and at least one interface in a non-backbone area. The ABR only uses summary LSAs received from Area 0.

You could have an area, e.g. Area 1, which has multiple ABRs. Area 1 will get summary LSAs from Area 0 through all the ABRs, and the Area 1 routers will send those summary LSAs to all the routers in Area 1, including all the ABRs. Remember, the ABRs have interfaces in Area 1, too, and are Area 1 routers for those interfaces. The ABRs will not use the summary LSAs coming from the Area 1 interfaces.

A non-backbone area should not generate summary LSAs. What number 1 is saying is that any summary LSA coming from a non-backbone area is ignored. An ABR will have at least one interface in Area 0 (backbone), and at least one interface in a non-backbone area. The ABR only uses summary LSAs received from Area 0.

You could have an area, e.g. Area 1, which has multiple ABRs. Area 1 will get summary LSAs from Area 0 through all the ABRs, and the Area 1 routers will send those summary LSAs to all the routers in Area 1, including all the ABRs. Remember, the ABRs have interfaces in Area 1, too, and are Area 1 routers for those interfaces. The ABRs will not use the summary LSAs coming from the Area 1 interfaces.