Not true, on both counts.
Run differential by itself only tells you if a team score more than it's opposition. Now, if a team scores 1200 runs and lead the league in run differential at 200, that's a good thing, but not as big a deal as if they only scored 800 and did the same. What I mean to say is, with any given run differential, the lower the scoring environment, the better for the team maintaining that differential.
So, since the Angels lead the league in run diff, and were 3rd in ERA, that's pretty big. They outperformed their opposition better than anyone that year. Their pythag record was the best in baseball. Yes, they were the best team going into the playoffs.
And guess what, so were the Red Sox in 2004. The Yankees overperformed their pythag by a whopping 12 games that year.
RD, by itself isn't everything, so you were right there, but when you look at it in the context of pythagorean record, it's one of the better predictive measures of success.