It's the Northern accent. The letter "r" is one of the most special letters in Vietnamese because it has different sounds in different dialects. In standard phonology and Central dialects, "r" represents /ɾ/ (common now) or /ʐ/ (traditional). In most of Northern dialects, it's /z/ (like the cluster "gi") while in most of Southern dialects, it's /ɣ/ (like the letter "g").
Maybe that is an effort of the speaker to pronounce the standard sound of "r". People tend to pronounce the standard sounds of letters in formal situations or situations that need the exactness. But in daily conversations, they use the distinct sounds of their dialects.
In this course, the letter "r" is pronounce as /z/ because the voice of the speaker is Hanoi voice. There is just a few sentences that have "r" pronounced as /ɾ/. I'll note these. Thank you.
It is one of the more annoying parts of learning Vietnamese that these regional variants are so acute. Like the how "V" turns into a /y/ here in the south with half the people you talk to but not for the other half. As I live on hung Vuong street I can only speak it intelligibly 50% of time depending on my listener