We agree that spreadbetters don't really come into it unless they hedge because, as you say, they can simply take an opposite view or net off against other clients.
The direct access thing basically comes down to timescale.
When people talk of "a mother of a short squeeze" are they referring to a large price movement to reverse a drop of several days/weeks ago or a few pence of drop from the last day or two, in which case I concede that what you are saying might come into play.