Step 23 has two parts; delete the blacks and dark grays, then delete the rest of the layer that is floating over the ocean. This leaves the lighter grays that are on top of the land.
You're omitting something while reading so I'll recap it for you and be as descriptive as possible.
1. You're on the Mountains layer. Foreground color is black and background color is white.
2. At the top of the screen click on the word "Select" then, in the scroll down menu, choose Color Range. This will bring up a pop up window with the foreground color selected as default. Set the Fuzziness slider at maximum, 200. This selects the blacks and dark grays.
3. On the keyboard hit the Delete key.
4. On the keyboard hit Ctrl+D to deselect.
5. This cuts the dark grays and blacks out of the whole layer, leaving the lighter colors. Now we have to get rid of those lighter colors that are floating over the ocean.
6. On the layer stack, ctrl+click on the Base layer to load it as a selection.
7. At the top of the screen click on Select then choose Modify then choose Contract then set the number at 20. This reduces the selection shape and brings it inside of the land outline.
8. At the top of the screen click on Select then choose Feather then set the number at 20. This "blurs" the selection edge 20 pixels.
9. At the top of the screen click on Select then choose Inverse. This selects the "ocean" area and not the "land" area but because we feathered the selection it will include some of the mountains on the "land" area. We want this to erase mountains that run right up to the beach.
10. On the keyboard hit the Delete key then hit Ctrl+D to deselect. Now the mountains that were floating over the ocean are gone as well as the ones that go right up to the beach.