Is this really true? Let's look at "more opportunities". A scalper can't watch tick charts on 30,000 securities. A long-term trader can follow every single exchange-traded asset in the world. So whilst the scalper may have far more tradeable moves *per security*, a long-term trader may have far more tradeable moves in total, because his universe of stocks/markets is many times greater.
Furthermore, issues such as overall market sentiment, valuation, and fundamentals do not have much of an effect on the second-by-second scalping timeframe. On the long-term timeframe they are significant. Therefore a long-term trader has more tools at his disposal than the scalper. Take something like convertible arbitrage or value investing - you can't simply cannot trade that on a minute by minute timeframe.
Now onto the allegedly lower risk. Scalpers are trying to exploit tiny percentage moves, and most use leverage, some of it significant. Position traders, on the other hand, rarely ever have even 100% of their cash in one asset or market. They might put 5-10% into one given stock, and maybe 50% into an entire market. If a stock gets halted and halves or goes up 50% on reopen, a scalper using no leverage at all will be down 50% if he is caught on the wrong side. A position trader might lose 2.5-5% on the same news. If there's a surprise rate cut or terrorist attack, the S&p could go up or down 10% in a few seconds, enough to wipe out many scalpers and seriously damage others. A position trader might lose 5% if there's a gap move like that.
Scalpers regularly have a much greater market exposure as a % of their assets, therefore they are much more at risk from black swan events IMO.
Finally, we have to consider the psychology risk. I've seen scalpers and daytraders lose their cool and go bust in one day. I've never seen a position trader lose their cool and go bust in one day - it usually takes days, weeks, or months. There is plenty of time to sleep on it, and come in the next day in a calm state.