In other words, Plumstead is simply a very heavily used station (I just checked the figures - 1.5 M entrances and exits, comparable to Charlton, and far more than the other ungated stations along that stretch of line: Woolwich Dockyard/Belvedere/Erith/Slade Green). Therefore, even if the proportion of fare evaders in the same, a blockade should catch proportionately more people. Also worth pointing out that Plumstead is very close to, and therefore shares much of the same catchment area as, Woolwich Arsenal. It therefore probably picks up a fair few fare evaders who could use either station but who go to Plumstead specifically because it's ungated.
I don't think that's quite correct. If you're travelling from Thamesmead, then I would say the main routes - the 301 and 472 - both stop nearer to Plumstead station than to Woolwich Arsenal station (they stop at the public market, then Beresford Square). Add that to that buses can be quite slow between Plumstead and Woolwich, and I'd expect it to be quicker to get the train at Plumstead - and in rush hours you're more likely to get a seat.
Coming back towards Thamesmead, the situation is different because the nearest bus stop to Plumstead station towards Thamesmead is Plumstead bus garage - enough of a walk to probably make changing at Woolwich better.
(Obivously, if you want the DLR, not national rail, then you're going to change at Woolwich in both directions)