I wonder if their notice is not absolute nonsense. They talk about breaches of their terms of service, which I think can be found here: https://go.he.services/tc/V1/en_GB/tc.html

The terms of service do purport to prohibit 'reverse engineering' of the app, which I think the developer receiving the notice may have done to understand the protocol between Haier's service and the app. However, it looks like the developer is in Germany, and did the reverse engineering for the purpose of creating something that, in a way, competes with the app. According to https://www.twobirds.com/en/insights/2020/germany/vertraglicher-ausschluss-von-reverse-engineering, contractual provisions in Germany designed to prevent reverse engineering to create a competing independent program after the original is already available to the public are not valid.

Maybe they are saying that the developer is unlawfully interfering with their business by inducing others to breach the contract. However, the terms of service don't appear to say prohibit connecting to Haier's services from a competing act (at least nothing in them I can find).

They don't really clearly define what their problem / claimed cause of action is. Maybe this is just an intimidation tactic against something they don't like, but they have no real legal case - in which case perhaps the community around it could band together to create a legal defence fund, and have Haier laughed out of court.

Disclaimer: Not intended as legal advice.

Edit: And better yet would be if they could find a way to intercept the traffic between the devices and Haier and replace Haier in that protocol. Then there is no option for Haier to try to restrict who can use the servers on their side. I assume the devices have a set of Certificate Authorities they trust, and it is not possible to get a trusted certificate without modifying the device somehow though.