Not being disingenuous at all, Gromit.
Jake has consistently argued in the past that an ever-increasing population, mainly fuelled by immigration, is the only way to reverse the UK’s relative penury (and hence increase its prosperity). He has argued that vital services and industries cannot function without such an influx. He has argued in particular (as you mention) about the need to increase the population, again mainly by means of immigration, in order to fund pensions. You may also have noticed that I have consistently argued that this is the economics of the madhouse and treats the State pension scheme as a gigantic Ponzi scheme (which it is, but there is no need to exacerbate that situation).
But now he puts forward an argument that a nation with a relatively small population and which happens to be relatively prosperous (and who does not, apparently, need vast numbers of immigrants to maintain that prosperity) is in that position precisely because it has a small population.
Hence my confusion.