Going on from Quinn's post, you must also remember that at the time, indirect injection was the only game in town with the technology of the day. Many indirect swirl systems were patented, Acro, Whirlpool, Ricardo Comet etc., once injector technology had reached a higher level of pressure and atomisation, the need for the higher compression ratio vanished, followed eventually by direct injection. Lister and others stuck with indirect injection until very late in the day.
The other added help was the cold-starting position on the injection pump rack which helped a lot.
The side benefit if indirect injection is the comparatively 'quiet' firing, something that was a problem with direct injection diesels for a long time, especially small, high-compression automotive engines with high advance positions on the fuel pump.
Peter