“Jesus is King” falls into the same bucket here. You’re trying to broadcast a particular belief or belief system (which is a particular brand of Christianity), with the eventual goal of converting/convincing the audience to your side. If this flag came out in a race in the middle east, I don’t doubt that the audience there would see it as hostile.
Let’s break it down a moment, hopefully in a way that doesn’t get this discussion banished to the other side of the forum - The flag asserts that Jesus, a first century carpenter, is the sovereign ruler of something. Originally, this would have been in the context of the 1st century Jews looking to overthrow those pesky Romans (“apart from the aqueducts, what have they done for us?”) and set up (what would have then been) a renewed state of Israel. However, this would have been since widened to include his sovereignty over all of creation post-resurrection, and ascension. It is making a statement about the world as the bearer (Hoel) believes it is, given his platform (in this case his high finishing position, but elsewhere on his IG) to be absorbed by the triathlon community.
It is a specific declaration of a particular belief of the world - one that comes into conflict with, well, let’s face it, a majority of humans both living and dead.
And so coming back to the definition of propaganda (per Merriam Webster) - “ ideas or statements that are often false or exaggerated and that are spread in order to help a cause, a political leader, a government, etc.”
They key here is that it doesn’t need to be false (or you believe it to be false) or exaggerated, just that they’re spread to help a cause, political leader, or government. And so, this is different than a banner that would have said “I am a Christian” or some statement about himself. The declaration of someone as king (or “King”) is always done so for a particular political cause, and in this case a religious one as well.
(and not that it should matter but since people’s personal beliefs are apparently relevant in this discussion, I’m an atheist, but spent a good chunk of my teens/early 20’s as an Evangelical, even going so far as to get a few degrees in theology/religious studies)