They are. I should know. I've worked as one for years including big tech companies. For e.g. FB has a lower pay than SWE, lower RSUs etc. and you can only get SWE pay if you transition into one, and that requires you to go through an interview process internally.
You are right in the sense that if you look at average SWE salaries and data engineering salaries, the average salary is higher for data engineers. Because the starting salaries for data engineers tend to be higher because of all the skills that are needed and there's plenty of SWE positions that require more than just a degree in CS. But if you start comparing salaries at maybe a senior level (4-5 yoe+), the salaries for SWEs start becoming a lot more than DEs. And again, I've worked in different companies, big and small, this holds true for all companies that have "Data Engineer" titles. There's of course companies like Netflix where you are a data engineer but still get a SWE title and get paid the same.
You're right, but most companies do not have those positions formalized and therefore you're expected to do those as part of your job, but not gain the financial benefits. Also, there is a big disparity in these titles and what the duties entail, which inherently again feeds into the problem.
AFAIK a "data engineer" at FB specifically is not quite equivalent to "software engineer specializing in distributed systems" which this article refers to--i.e. there is a totally separate job track for SWEs in general. The "data engineer" job title is definitely not standardized between companies.
