Shouldn't this cause a fire? This website shows a telescope projecting the sun onto a blackboard:  https://astronomyconnect.com/forums/articles/2-three-ways-to-safely-observe-the-sun.21/
Why isn't the board catching fire?   You can easily start a fire on a sunny day by targeting the focal point of a magnifying glass onto something flammable.  Why isn't the telescope in this picture doing the same thing?

Photo by Luis Fernández García
It could start a fire if the screen is at the focal point of the optical system. That is how you light fires with a magnifying glass.
Here, the blackboard is likely away from the focal point, so you can see the shape of the eclipse (and you get a bigger image) without setting things on fire.
Although this is fairly safe, there are a few things to pay attention to:

If you do this, make sure nobody can walk between the telescope and the screen, because if they go near the focal point, they could get very hot.
Doing this will cause your telescope to heat up. If there are any plastic parts, they can melt.
The telescope in the picture seems to have a small opening. Don't do this with a big telescope. You don't need to collect a lot of light.


Not an answer to the question, but an important note: Observing the Sun is the most hazardous thing you can do in astronomy. Make sure you know what you are doing before you try.