What's false about the assumptions? He's talking about flight at the same
angle of attack at different altitudes.

Are you serous?
First, take a look at his opening statement.We dont fly planes like
this in real life.It seems he has made the deductions first, and then
came up with the opening statement.Also, not all of these deductions
can be true at the same time.

The purpose of that section is not an operational guide. It's merely to
illustrate a point -- that being the effect of flying at the same angle of
attack at different altitudes. This leads to the last point: to do that you
need more power.

I don't see which one of the deductions isn't true given the parameters.