It’s a bit disappointing that an article purporting to educate us about something we should know but may be unaware of, is completely wrong about the example of how an airplane climbs - you don’t climb by pitching up, you have to increase power.
In a glider you can't do that so there when in level flight you have a limited amount of forward momentum available to help you climb if the air itself isn't moving up, you are continuously trading altitude for speed and vv (easy to see in a dive: everybody expects you to gain speed in a dive because can all relate so something falling, it's obvious the reverse has to happen when you climb and the stall speed is a design parameter of the aircraft at a given altitude combined with a bunch of other factors).
https://aviation.stackexchange.com/questions/27693/how-does-...
If you pitch up you can only exchange speed for a little bit of altitude, briefly.