Yes, all arbitraging activity is an entrepreneurial investment. Scalpers can and do suffer losses periodically. No matter how small the time element involved in an arbitrage or how seemingly secure the pending sale might be, losses could ensue from arbitrage. Of course, Mises is not referring to that point in the quote. He is making a contrast between situations in which a person could earn a profit or suffer a loss from an investment, e.g., a capitalist-entrepreneur in the market, and situations in which a person can only earn a profit while the loss is suffered by someone else, e.g., a bureaucrat in the government.
Walter Block lists ticket scalpers in his rogues gallery (Ch. 12):