A secular hospital chaplain will follow the guidelines laid down by his bishop and diocese. If he’s religious, he will follow the norms of his constitution.
Respecting the verity of the hour (or the “truth” of the hour) is a principle laid down by the Council (SC 88) and the General Instruction. This can be loosely interpreted to mean that one can pray Morning Prayer as late as 11 AM, since it’s still morning. But 1 PM is not morning, so praying Morning Prayer does not respect the verity of the hour. More strictly, it is to be said early or upon rising. In monastic settings, it’s the dawn prayer, with Midmorning Prayer being the prayer when the sun is already in the sky in the morning hours, after breakfast.
“Principles of the liturgy” are those concepts and principles that govern liturgical prayer, and this includes the pronouncements of the Church. This includes constitutions, general instructions, particular instructions and theological concepts.
Praying Morning Prayer in the afternoon is neither sinful nor disrespectful. At worst, it is “out of character”, and since respecting the verity of the hours is a principle the Council upheld, saying Midday Prayer makes more sense.