Why is multiple independent index always better than concatenated index
Original author here.

The quote is meant as following:

In your first query, just filtering on B and C, what would be better:

Indexing 1: Index B, Index C
Indexing 2: Index (B,C)


Clearly, Indexing 2 is better.

The context was meant to be: if I index all columns, is it better to have one index with all of them, or one separate index for every column. In that case, it is almost always better to have one with all columns.

It's a design advice, not for assessment.