Great explanation, I really get this now, thanks!
The below from Eigrp module
EIGRP will check if you have any subnets that fall within the range of your network command(s) and if so, it will advertise the classful A, B or C network to its neighbors. It will only summarize routes from subnets on its own interfaces, not the ones you learn from other routers.
In this article you are saying Eigrp is classless
This statement is found within the following lesson and is explaining how auto-summary functions for EIGRP:
So when auto-summary is enabled, this is the behaviour that we see from EIGRP. It will advertise the classful subnets that correspond to the network commands. Note here that Rene mentioned that:
…EIGRP is not going to automatically create the most optimal summary routes for you.
The purpose of the lesson is to show how auto-summary behaves so that you can either use it correctly or avoid using it and employ a summarization technique that will better suit your network.
When auto-summary is enabled, yes, EIGRP will behave like a classful routing protocol. But when disabled, it is a classless routing protocol.
I hope this has been helpful!