Hi Sergey,
I'm not sure I understand exactly what part of the advice on that page you are having issues with? In general, a database engine will use an index if the column is indexed, unless it's compare with a LIKE and there's a "%" at the start. Both Oracle and MS SQL are pretty good at issuing query plans that make optimal use of indexes.
The only difference between using variables and using hard-coded values is that for hard-coded values, the database engine may have to build a new query plan, whereas it can reuse an existing one if you use variables (though depending on the nature of the variables' contents, that can be benificial or not).
A database engine should only use full table scans if a column isn't indexed, or a LIKE with a starting "%" is used, and there are no other ways to restrict the data. I don't see any advice on that page that would lead to intentionally causing these situations.