Along with Luxemburg, De Leon wasn't a Left Comm either. Both of them died before Left Communism emerged from the 3rd Int.
That whole conception it seems to me comes from wikipedia not the real history of the emergence of the Dutch/German Left currents. There's a tendency I think to conflate the left of the 2nd Int (even though I don't think De Leon and the the SLP were even in the 2nd Int) with the left of the 3rd Int (because for example Pannekoek was part of both) and call anyone on the left of the 2nd Int a 'Left Communist'.
Not really. I think this whole quote is misleading. First, Luxemburg was never a Left Communist - simply because the term itself invented after her death. But even when she was alive, future LCs like Pannekoek were already anti-parliamentarian and in opposition to her - just as the slight majority of the German CP-Spartakist. Moreover, neither Luxemburg nor LCs were against a Revolutionary Party. They just have different conceptions of it. Needless to say LCs and Luxemburg did not advocated participation in government but the soviet power. The union question is more complex but LCs also rejected unions tactically.