Are the other causes of an earthquake (e.g. volcanic eruptions) the cause of the "sudden slip" which is the main cause of an earthquake?
When you look at a table of all the major historic earthquakes they are all related to plate tectonics, either directly or indirectly. These are all of the magnitude 7 to 9 events, and most of the magnitude 4 to 6 events. The linkage with volcanic eruptions is real, but weak. Again, most but not all, of the world's volcanoes are generated by plate tectonic processes, the main exceptions being intra-plate movement over mantle hot spots. As we get down to lesser magnitudes we are getting into greater frequencies. It is roughly a logarithmic relation ship, so for every major earthquake, there are thousands of low to medium level earthquakes and millions of small micro-seismic events - the latter being so small that nobody notices - except for geophysicists. There is a host of processes which give rise to low-level seismicity, such as submarine slope failures, turbidites, sedimentary compaction, thermal contraction, magma redistribution at depth, erosive load shedding, after-shock adjustments, landslides, mudslides, rock-falls, rapid draw-down in reservoirs, mining subsidence, fracking, and probably a load of other stuff that I can't think of right now. 

So it's roughly a matter of scale. The USGS is correct that serious earthquakes are all caused by plate tectonics. On the other hand, your other source was right, that some seismicity can be caused in a multitude of ways. 

Afterthought: Probably someone will point out very rare exceptions, such as major bolides crashing into Earth and causing shock waves right through the planet. But such are extremely abnormal.