Yeah, the "snow-how" punch-line was that the airport "never" closes due to snow.
Still, ffay005 has a point. As soon as it was turned into marketing, the advantage was exaggerated and thus the marketeer created expectations like 'HEL will always be better in terms of interruptions due to weather'. While it is likely better than LHR at handling snow, the airport is nothing exceptional. It is not the pinnacle in snow handling and it certainly has its fair shares of delays.
It can be dangerous using exceptionalism in marketing. "Shortest route to Asia" *) was a truth no one thought could change, and yet here we are.
*) Not saying it was a bad idea using that one - I think it was the main advantage of Finnair and of course marketeers should use it. Just saying that when you lean very heavily on one single thing, there is a serious backlash when it's gone.