Actually I've checked up, and the company for whom the adverts were written wasn't actually the one Charles Dickens had worked for as a boy, but another rival one with a similar name. The place he worked in, and evidently loathed, had been set up by the brother of the man who ran the company who ran the adverts, following some kind of quarrel.

The implication of this would be that this poem/song does not indicate that Dickens really felt quite friendly towards the place he'd been employed, in spite of what he said later. Writing this ad copy would have been a way of getting his own back, by helping the competition. (As well as making a few badly needed guineas in the process.)