Basic question about phylogenetic trees
Yes, your understanding is correct.

That being said, it is standard to refer to population / species at specific points in time. Also, it is standard to refer to these ancestral populations relative to what they are the ancestors of. For example, we generally don't refer to the population 2-3 (as you named it) but we rather refer to the node/population/species 3 as the Most Recent Common Ancestor (MRCA) of Athropoda, Mollusca, Chordata and Echinodermata. The tree presented here is not complete but all Athropoda, Mollusca, Chordata and Echinodermata are Bilateria while Cnideria and Porifera aren't. Hence, you could say that the ancestor at node 3 is the MRCA of all Bilateria.

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