Ende Gelände has first been a nonsense-rhyme. It is used to express something like Game Over or Your time is up now. and also This is enough. As Christian Geiselmann mentioned in a comment, the rhyme is just used to emphasize the word it rhymes with, so the function of the rhyme word is merely phonetical and not semantical at all. Analogue cases are the fixed phrases Hätte, hätte, Fahrradkette (literally translating to would have, would have, bicycle chain) and Aus die Maus (literally: off the mouse). They don't make any sense, but the rhyme underlines the word which should be emphasised.
Anti-coal activists used it as the name of their movement, because Gelände means terrain. So, the name of the anti-coal movement (which the website you found refers to), is expressing that there is no more terrain left to waste for coal mining. I think, they just used the coincidence that the former nonsense-rhyme can also be understood to express the meaning they want to deliver: they want to make coal mining an end. Of course, the meaning Time is up or Game Over is still present in that creative use of the phrase, and this is definitely intended.