I did a bit more reading up on it and think you may well be right, although it's still possible to put some into a pension, as that would be seen as a reasonable use of funds to plan for retirement.
Apparently, the test for deprivation is a subjective one, so dumping the whole inheritance into a pension would be seen as notional capital for housing benefit, but a smaller amount possibly not. It would be up to the authorities (and eventually a judge if necessary) to decide.
Sorry if I came across as a tad mansplainy