The simple reference was in direct response to Essex asking whether he would have been arrested if he had a jacket over the shirt.
In order for this offence to be valid then the writing in question needs to be displayed, and that was the point being made.
As for whether something is deemed as threatening or abusive is subjective. There is the caveat that this also be likely to cause harassment, alarm or distress. So it's not just about being offensive, it's about doing so in a way that could cause someone to be seriously affected as a result of what you have said or written. And that is why any charge needs to be tested in court to see if it exceeds these thresholds.
I do feel that many people are too easily offended, especially about things that have absolutely no impact on their lives or the lives of people they know. But equally there are also some who look to provoke a response by being offensive beyond all normal social boundaries. And there is a lot of grey in trying to police that.
But mocking the death of innocent football fans, including many children, is on a very different level to calling an environmental lobby group members Nimbies etc. So I'm not entirely sure what point you're looking to make.