We don't think we would blame Boris for the creation of the Euromyth

Something of a stretch here, a claim that Boris Johnson invented the very genre of Euromyth and the enthusiastic pursuit of that by others is what has led to Brexit:

Even worse, Johnson created a school of EU reporting: the entire British press, to varying degrees, began peddling Euromyths, fuelling the kind of Europhobia that no UK politician dared to stand up to, and which ultimately has now led to Brexit.

No, we don't think that can be allowed to stand really. For two reasons, the first being that it wasn't those making the anti-EU stories such fun which started the game. Rather, it was the absurd claims of the federasts which set things off. 

The European Union has created peace in Europe? a regular claim, as we know, that an organisation which came into being in 1992 created peace since 1945... and rather missing the influence of NATO there. The euro would produce a burst of economic growth? Err, yes, let's brush that one under the carpet shall we? The more current insistence upon an FTT to raise revenue when the Commission's own report into it insists that tax revenue will fall as a result of the FTT causing a smaller economy? 

The second reason would be that the Euromyths all too often turn out not to have just a grain of truth to them but are actually the truth. It really is illegal, a criminal offence punishable by 6 months inside and or a £5,000 fine, to sell bananas of excessive curvature for direct human consumption. Or to market apricot marmalade. These are not myths they are truths. And there really is a law, the jams, jellies, extra jellies and marmalades regulations which insist that it is legal to make jam (or extra jam) from tomatoes or carrots but not from  marrows or turnips. At the pain of that 6 months and or five large again.

Who needs myths when you've bureaucrats trying to rule 500 million people in that manner?

No, Johnson did not invent the Euromyth nor did myths of any kind lead to Brexit. The system itself created the stories and the system itself led to our leaving. it, to borrow a phrase, tumbled from its own internal contradictions.