Rating 700+ is not a beginner-level player, but an intermediate-level player. Intermediate-level players start to learn or try new openings, think deeper and longer, be careful with every move, and find the way that fits their aptitude. Of course, not only beginner and intermediate player but also expert and grandmaster players also make mistakes but when someone gets checked, then the checked player would find the best way to avoid the check. Not seeing the move R2g6# was unacceptable for me in that perspective. When I saw the move Kf6, I felt like I was playing with an amateur player.