Schema Design Anti Patterns Part 1 Massive Number Of Collections

Hello All thanks for the help, I am new on mongo db and I was studying the Schema Design Anti-Patterns Schema Design Anti-Patterns - Part 1 - YouTube at the beginning of the video she explained why is not a good reason to use endless array into a Collection. However in the second part of the video which explained about Massive number of collections at least for my perspective she did the endless array:

So I am wondering if that is correct or if I understand this incorrectly.

Thanks in advance