Not a lawyer; would this likely stand up in court?
I’m not a lawyer either, but I don’t think so.
The developer of this Home Assistant integration is German. European law allows people to reverse engineer apps for the purpose of interoperability (Article 6 of the EU software directive), so observation of the app’s behaviour or even disassembling it to create a Home Assistant integration is not illegal.
In general, writing your own code by observing the inputs to and outputs from an existing system is not illegal, which is for example how video game emulators are legal (just talking about the emulator code itself, not the content you use with it).
If it’s a Terms of Service violation, it’d be the users that are violating the ToS, not the developer. In theory, the Home Assistant integration could have been developed without ever running the app or agreeing to Haier’s Terms of Service, for example if the app is decompiled and the API client code is viewed (which again is allowed by the EU software directive if the sole purpose is for interoperability).
The code in this repo is likely original Python code that was written without using any of Haier’s code and without bypassing any sort of copy protection, so it’s not a DMCA infringement either.