Have you honestly never been in a situation in which you felt absolutely certain you were correct about something which you were in fact completely wrong about, despite the passionate arguments of someone else? Have you never been utterly convinced that the other party was basing their entire argument on misinformation, when in reality you were the one who was misinformed?
Even police officers are only human and can fall prey to cognitive biases and mistaken judgement. Sometimes that means not believing someone who it turns out actually is correct. That's not bad behavior, that's just bad judgement.
In the absence of ready access to the exact wording and legal interpretation of the law, what is a police officer reasonably going to trust more? The word of a cyclist they've never met who might very well just be trying to get out paying a ticket? Or their own judgement reinforced by their own experience and authority as a law enforcement officer?
I highly doubt the officer believed Mr. Ionescu was lying - I think the officer merely believed Mr. Ionescu was mistaken about the law, and that they personally were not.
Could the officer have contacted someone and verified the law? Perhaps - although typical police procedure as I understand it is that you cannot detain someone without cause, effectively meaning that you have to immediately choose between citing them for something, or letting them off scot free.
As far as I'm aware, the police aren't really allowed to witness something they believe to be a crime or infraction and then hesitate in citing it in order to double check the exact text of the law to be sure. An officer can't just show up at your doorstop and say "I saw you run a red light last Thursday" and have any expectation of being taken seriously. But if they stop you right then and there and say "Hold it! I just saw you run a red light!", we all seem to collectively agree to take that seriously, even in the absence of other evidence.
Of course, even if the exact text of the pertinent law was available for immediate perusal and confirmation, you still run into issues like wording and interpretation. Unfortunately, this is where different people's understandings of a law can really begin to radically differ, and it typically isn't simple or easy to discern who has the right of the issue.
... which is why we have lawyers.
The officer in this situation seems to have had only two options - trust their own judgement as a law enforcement officer and levy a fine, or trust Mr. Ionescu's word as an untrained layperson and let the issue go.
If the officer let Mr. Ionescu go and was wrong to do so, then they just failed in their duties to enforce the law and the situation cannot be rectified later.
On the other hand, if the officer instead trusted their own judgement regarding the law and was wrong to do so, then the fine could still be contested and the courts would sort the matter out - which is exactly what happened, but is also exactly why we even have a court system in the first place.
Making a mistake in judgement in the course of fulfilling the duties of your office doesn't make you an asshole. Falling prey to cognitive biases and improperly valuing your own knowledge compared to that of others doesn't make you an asshole. Sticking to your guns (in the temporary absence of concrete evidence confirming the issue either way) despite the insistance of someone else who ends up being proven right doesn't make you an asshole.
It just makes you a human.