Sure, never assume malice when it could be just ignorance/stupidity. But at the end of the day, this is a major product and service ($33M in VC funding so far [0]). Even though the author tweets (and includes the screencap) "Superhuman is a surveillance tool that intentionally violates privacy...", my impression overall was that his main complaint was about a company's culture and ethical decision-making process:
> Second, I want to talk about why this particular issue is so important. Not why privacy is important; we are all already learning that the hard way. Rather, why making ethical decisions at the earliest stages of your company is important.
> When a company first forms, there are no norms or principles guiding how its people should make decisions. It’s basically just what’s in the founders’ heads. With each decision a company makes, its “decision genome” is established and subsequently hardened...
In other words, if this is a decision Superhuman made early on and embedded into its main product, then, the author argues, we should be very wary of its culture, and how it will weigh-in on privacy issues going forward. It's from this claim that the author seems to have built his anger on.
Read receipts have been a feature of email and messaging clients for at least more than a decade now. Among the tech-literate, the tradeoffs and potential problems/controversies are well-known, such that opting-in (or out) of read receipts is a near-ubiquitous option in every service's account settings. I'm sorry, but it really stretches plausibility to think that Superhuman put in as little thought about it as you propose.
[0] https://www.crunchbase.com/organization/superhuman#section-o...