They get confused because from the outside they look the same: a person is not doing what they meant to do.
The difference is inside, and it matters. Laziness is when the task is not needed, and the body honestly refuses to spend on it. Tiredness is when it is needed and there is nothing left to spend.
One question separates them: what happens when the task is cancelled. With laziness there is relief, sometimes immediately. With tiredness there is nothing, because it was never about the task.
The strategies are opposite. The first wants a conversation about why this at all: it often turns out the task belongs to someone else, or was needed six months ago and is now coasting. The second wants recovery, and no amount of discipline turns the second into the first.
The mistake is almost always the same one: tiredness gets called laziness. Then it is treated with pressure — a plan, an early alarm, a promise to yourself. Pressure eats what is left, and a month later the person cannot do even what they could before.
The reverse mistake is rarer, because admitting a task is unnecessary is scarier than admitting you are tired. An unnecessary task drags a question behind it: why did you take it on.
In practice, rest tells them apart. Rest cures tiredness and does not cure laziness: after three days off, an unnecessary task is exactly as unnecessary, except now you have no excuse.