SEO, keyword density and Google penalty?

Here is a very interesting topic. A long time ago, I saw this video from Matt Cutts, webspam team lead at Google:

first of all, he said that the importance of keywords decreased depending on the number of occurrences in the text. This is how the algorithm works and it’s ok.

But then, he implied that many keywords would lead to keyword stuffing without being precise. When I saw this video for the first time, I got scared because I thought that we could have a penalty if we would repeat words several times. Today, I understand that this wasn’t true. In fact, if you discuss the reliability of a Chevrolet Impala, you aren’t talking about an African animal and you have to mention “Chevrolet Impala” every time in a text. If you take seriously what Matt Cutts says in the video, you may think you should use alternative names for the Chevrolet Impala like “chevy”, “family car” but then will Google is clever enough to understand that we talk about the same thing? I think that in many cases Google can’t understand the link between expressions. 

Real world example:

Those two pages below are among the best ranked pages for ‘Iphone purchase’ from the google search engine.

This page about Iphone counts 16 occurences of the keyword https://www.macobserver.com/tips/quick-tip/check-apple-store-iphone-x-stock/

This page has 25 occurences of the keyword Iphone

To conclude, I think this would be a mistake to try to diversify the wordings to avoid keyword stuffing. We haven’t seen any keyword stuffing penalty.