Subselects are almost always better when it comes to optimization.

The author is really amateur. His inability to do what amounts to SQL I in college is quite telling, and somehow hes opinionated enough to write a huge blog article about it. I like the analysis and diagnostics of the query planning - but his query writing is trashgutter.

    SELECT c,d FROM t WHERE a=(SELECT MAX(a) WHERE b=X from t) 

That query is what I would have done from the getgo. It reads much better and is going to pretty much always be the optimized query plan. This is basic stuff...