You're absolutely right about that statement. They aren't. In fact those who consider themselves to belong to one party or the other will swallow all kinds of bs just to maintain their ill placed loyalty. For instance, you'll see right wing news stories about how the republicans handle the national debt much better than the democrats. This is false. During Clinton's and Obama's terms we actually got to a surplus state. We were no longer borrowing money to pay the National Debt. Before George Jr's term had become a year old we were back to deficit spending. The same happened when Trump took office.
Some people have "their" party so ingrained in their brain they will cling to them no matter what.
Sure, but the places that matter in an election, especially a close one, are typically districts that swing back and forth and are not entirely locked into a numbers game. If they were, elections would not have a pendulum like quality they do now where the Democrats and Republicans switch locations in the power structure of the government.
I'd also like to point out that saying 'SOME people' when talking about die-hards does not in anyway invalidate the notion that there are other people who are not die-hards for their parties and that this plays a distinct role in predicting close races.