There is no way to answer what happened in a security breach until way after it has occurred.
Western Digital has very little information on their user identity unlike Google, Amazon, MSFT, FB etc. Even if user data had been leaked, there is very little consequences to user privacy. There is zero evidence that the data on the My Cloud Home itself has ever been compromised. I have seen posts of yours where you brought up incidence of data breach on devices that were EOL (End of LIfe) years after they became unsupported by WDC which is not the case here.
I would suggest every MCH owner monitor unauthorized access to their MCH devices through their firewall such as their router should they decide to have their MCH remained connected to the internet. But so far there has been zero suspicious activity on all my MCH devices being monitored.
