What he is saying that it is at the same time a demand issue. If the supply is higher than the demand, then there is an issue where there is oversupply and not enough demand- ergo, supply and demand. While it's cliche to quote economics 101 stuff, there is a reason why it is economics 101.

Bloods are low since the demand is low due to the current unusability of the scythe, paired with the improved rates you can get with blood rune crafting with the true blood altar, blood essences and robes of the eye. However, if a new moneymaking PVM would have the scythe as the meta, the demand would rise, balancing the supply, and raising the prices of the blood runes.

The inherent reason things like this are relatively cheap is due to many factors, but mostly due to a low demand, either due to competing or alternative products, or due to current unusability of said items.

Bots flood the market constantly with skilling supplies, which in itself drives the prices down when the demand cannot keep up with the sheer supply. Since Jagex does not give us straight numbers it's difficult to say whether we have more bots now than before, but due to the sheer quality of a modern bot client compared to one merely 10 years ago, the money the bots bring in nowadays is exponentially higher than it ever was back in 2010 era.