I think the remark was made in the context of being the first Royal Flight where the Queens Flight could have done the job. Didn't they have Vikings at the time that could have taken her, albeit in a lot less comfort? Certainly the flights prior to the coronation didn't count as she wasn't crowned then.
Excerpt from wiki:
As The Queen's Flight from 1952, the unit operated a variety of aircraft for the transportation and pilot training of members of the Royal family, including Vickers Viking, Avro York, de Havilland Heron and Devon.
No long haul aircraft (I can't believe they would have taken her in the York!) so they had to rely on the airlines.