He notes that much residual gum remains on the stamps, and he has not been able to remove it. That's bad for collectors - who will need to use mounts or attach the stamps to pieces of clean, archival-quality paper - but good for people who want to re-use the stamps.
The joke of all this is that Royal Mail has wasted a great deal of money and effort in producing the security slits, as the stamps have enough gum on them to stick to another envelope. Since parcels in the UK no longer get the stamps cancelled and sometimes letters escape also, there will be plenty of reused stamps, despite Royal Mail's expense and trouble.
For more information on naptha, the name of the general class of flammable hydrocarbons, see the Wikipedia article.