Circuit direction of flow according to tutorial

I'm following an introduction in circuit electronics, and there seems to be a contradiction in its explanation of the direction electricity flows. First it says that electricity is the movement of valence (outer shell) electrons from atom to atom, and that the negative pole of a power source (being of like charge) will push electrons from the negative pole and electrons will flow from the negative pole to the positive pole, which attracts flowing electrons.

Then I've come across this description:

the positive side will have a higher voltage than the negative side, which is exactly what we want. In fact, when we measure voltage, we usually say that the negative side is 0 volts, and the positive side is however many volts the supply can provide.

What have we learned so far?
*Electricity will flow from a higher voltage to a lower voltage.
*DC voltage sources always have two sides, called positive and negative, with the positive side a higher voltage than the negative side.

The bolded part especially seems to contradict what was said earlier about the flow being from negative to positive.

Here is the link to the site

Could someone explain how the electricity in one explanation flows from negative to positive, and in the other explanation from positive to negative?