The Hive heating system can be used standalone with no hub - but in that case it operates as a “dumb” thermostat with none of the advantages of home automation. (This can actually be an advantage - for example, it will (should) carry on working in the event of a catastrophic internet failure.)
But, yes, in reality a cloud account is required; their cloud servers control your heating and HA is completely reliant on an unsupported API. AFAIK this is true for all vendors, not just Hive. I suppose there is some advantage to the vendors in continuing allowing access to their APIs - they get to collect/sell your data, keep the hobbyist/hacking community interested in their product and so on. But in principle any of these systems could be shut down in an instant. And I am not paying any monthly fee to Hive …
There is an alternative, I suppose - hooking up a Sonoff or Arduino to the relays that control your boiler heating circuits and then using Heaty/HA or something similar without any cloud involvement. (May need something a bit more sophisticated if you want to use Opentherm or something like that - which I believe is supported by Nest.) But that is a bit too far down the electrical engineering side for me, and Hive is ok. For now 