If they had gone ahead with this plan, Syriza would be isolated from everyone --- the Greek people, who would instantly lose the majority of their assets in the euro/drachma haircut, and every lender in the world that might help soften the landing.
That's why the Left Platform leadership had to talk about arresting their own central banker. They'd have been executing a financial coup and rewriting the rules of their economy without consultation with anyone else inside Greece. They'd have been deciding by themselves not just to exit the Eurozone, but to do so abruptly and without the cooperation or assistance of the ECB.
This isn't just about Greece severing ties with the Eurozone; it's about how they would have done it.
Edit
Here's a Greek expat trying to lay out what an orderly transition from Eurozone membership to a drachma economy would look like:
http://pavlos.geekhost.org/2015/07/05/what-grexit-looks-like...
He seems remarkably sanguine about the whole thing. He seems to think Greece only imports iPhones and other luxuries. In reality they import about a third of their food, they import electricity, oil, virtually all medicines. He mentions those are "cheap" but that's because the pharma industry was treating them as a charity case for the last six months.
Where I do agree is that Greece cannot radically increase exports simply by switching to their own currency and printing a lot of it. People aren't going to suddenly double their consumption of cheese if Greece halves their prices.
Your link simplifies to an extreme, hand-waving away all the difficult parts of implementing the plan (e.g. how to keep the banks open, how to keep the government functioning, what the response would be of the IMF and ECB, how to prevent capital from immediately fleeing en masse, whether Greece actually has capacity to print and distribute drachmas, etc.). Its suggested approach might or might not work out, but it definitely would not be any kind of smooth ride.
Perhaps he was just overly optimistic in the wake of the referendum No vote. His prediction – “it’s likely the institutions will seek a quick compromise to avoid serious damage to themselves and the Euro” – turned out to be fairly inaccurate. Instead they refused to compromise and in fact made even stronger demands than they had before.
What makes todays reports bombshells isn't the idea that Tsipras (more accurately, the Syriza Left Platform) had a Plan B involving the drachma. Approximately everyone who reported on the Greece/EU showdown noted that Plan B would somehow involve the restoration of the drachma.
The bombshell is that they had a plan to do it by fiat, extrajudicially, in the process seizing 10-15Bn EU from the Greek treasury in the hope that they could parcel the money --- which wasn't properly theirs --- to keep the government running during the transition.
The reports suggest Left Platform, Varoufakis, and Tsipras were planning on rewriting the entire Greek financial system, without any consultation with the Greek people beyond the agreement of the Left Platform faction.