Hello Fadi
In Rene’s statement, what he is saying is that when a level 1-2 router connects to another area, it will send level 1 LSPs to all of its level 1 neighbors in the same area.
Now according to this Cisco Documentation, it says:
A Level-2 IS that indicates that it has one or more Level-2 neighbors in other areas may be used by Level-1 devices in the same area as the path of last resort, also called the default route. The Level-2 IS indicates its attachment to other areas by setting an attached bit (ATT) in its Level-1 LSP 0.
Also the RFC 1142 for IS-IS states that:
…the Level 2 Decision Process informs the Level 1 Update Process of the values of the default routeing metric to and ID of the partition designated level 2 Intermediate system in each other partition of this area.
In other words, each L2 router will provide a metric for the default route, which will be used to determine which one will become the default route.
I hope this has been helpful!
Laz