While Cromwell's far from a great example of democracy.

He is the example of parliament creating a law that made killing a king illegal. And the very creation of our current constitutional monarchy. His actions basically created most of the constitution changes the nation now works on.

Hence why the example was made.

You are correct in the fact that telling secrets in parliament is technically legal. It is worth noting that parliament has the power to enforce rules upon itself. Technically to the point made by Cromwell.

IE in the event and MP was to announce secrets in parliament. Without gov approval and more so now it is televised. (This was not the case in my youth. When recording parliament was illegal for that very reason.)

Parliament would technically be able to have the MP imprisoned. Although as of now parliament has no where to store them. It was the tower of London in the past.

But yep it would have to be parliament that enforced such rules. And doing so would require a majority. Hence why bojo tried to close parliament and got prevented.