A non democratic country has fewer checks and balances on these things than the ones that do. But you are right, that's not a guarantee at all. Just a matter of degree.
In the US both tariffs and sanctions can be (and are) unilaterally imposed by the president.
Sanctions are crystal clear thanks to the IEEPA [1]. The president needs only declare an emergency, which is typically done in the exact same executive order imposing the sanctions. We're currently under at least 42 different national "emergencies", of which 34 are to impose sanctions. [2] The legal basis for tariffs is less clear, but in effect no different. Trump's tariffs [3] were all passed unilaterally, appealing to all sorts of acts that grant the president conditional tariff powers, but where the condition is framed in a broad enough way to include nearly any rationale.
But also consider that an act of Congress could modify IEEPA or repeal a specific tariff or sanction that the US president enacts. Perhaps that's a rare event, but those checks and balances are in place.
In China, Xi can decide to impose tariffs or sanctions, and there's nothing anyone can do about it.
I think this issue is what really cuts to the matter. China, on paper, has a similar system in place. Their system is based on tiered elections. You elect a local representative (who has significant power), who then elects (alongside other elected representatives) their representative. And this continues on up to the top where you have the ~3000 members of the National People's Congress who ultimately elect the president. The... NPC.
The NPC has immense power. They can override anything Xi Jinping does, and even have the power to completely recall him, or amend the constitution. But of course this won't happen. They have a one-party system with relatively minimal internal conflict. So even though the NPC is genuinely powerful and has every check and balance imaginable, they will not be using them.
