> What this means is that the sentences, “This is a car”, and, “This is the car”, would both be, 「これは車です」. There is no differentiation.

This is not always true. The latter could be 「これが車です」. The は and が particles are very similar but are still different. Fully grasping this small difference is one of the biggest problems Japanese learners encounter when studying grammar.

Closer to the beginning, the article also mentions:

> The topic of a Japanese sentence is very similar to what other languages refer to as the subject. The subject of a sentence is the person or thing that does the action described by the main verb in the sentence. These are, in fact, slightly different concepts, but for now, we will treat them as being the same so as to keep things simple.

It turns out that は marks the topic and が marks the subject. I feel that many times the confusion between は and が in Japanese learners happens because the learning material tries to make this simplification in the beginning. When it's time to learn が, it's hard to retrain the brain.