The data is not accurate. The stats are literal numbers, Which means these guys are not mentioning how many hard drives each vendor sold. But instead they are just comparing the Faulty drives numbers. If you consider the number of HGST and Seagate pieces sold then only these stats or demographics can be considered reasonable.
Punchline: This review or analysis could be biased.
They're not just comparing the faulty drives numbers. Maybe you misunderstood the way they presented the data: in the first table, the "Number of Drives" column refers to the total number of drives they studied for each model, not just the ones that failed. So they give the sample size and the rate of failure for each model.
Or are you saying that the difference in worldwide sales for each drive model somehow renders Backblaze's failure rates invalid? The size of the population is irrelevant unless the size of the sample exceeds a few percent of the total population you are examining.
By definition, it's going to be a true analysis of the drives when obtained the way Backblaze obtains them and when used in the environment Backblaze uses them. Both of those may be different from Joe Random consumer.
Punchline: This review or analysis could be biased.