‘...”So” as an interjection.’
The use of ‘so’ or a similar word as an interjection is very old in the English language. The very first word in the Old English poem “Beowulf” is ‘Hwat!’, which is used much like ‘So!’. Some times, poets and writers used ‘Lo!’ in the same way.
Some times, poets and writers used Lo! in the same way.
Some times, poets and writers used LoL in the same way.
I thought, “Who knew?”