I agree that the work versus leisure indifference curve is odd. Actually there is an undisclosed assumption that when you work more you make more money.
I see they tried to make something oversimplistic to show how indifference curves apply to anything, and not just consumption goods, but it does not really work.
Of course the idea of an indifference curve is not to just say that an individual would give up 1 unit of good A for one more unit of good B: the proportion is not 1 to 1 (it can be at some point of the curve but…)
The same should be true for the work versus leisure curve but the problem is:
1) they plot two points on the curve which make you think that the trade-off is 1 to 1: (6h leisure; 4h work) and (6h work; 4h leisure) (only the shape of the curve makes you understand that the trade-off is not 1 to 1 but somehow they prefered to plot these two, probably because for the reason I mention in my point 2)
2) obviously the total number of hours in the day is fixed so if you spend one less hour working you have to have one more hour leisure as a matter of fact. It does not mean you’d be indifferent between the two. But it just can’t be otherwise. Given this “constraint”, drawing an indifference curve between these two “goods” is total virtual.
Maybe we should think about this curve rather as an indifference curve between money and leisure, assuming money is linked to the number of hours you work. Then the question would be how many hours of leisure would you be able to give up to earn $100 more a day or so. That would make more sense.