YES

(format LCTRS :logic QF_LIA)
(fun ack 2 :sort (Int Int Int))

(rule (ack 0 n_0) (+ n_0 1) :guard (>= n_0 0) :vars ((n_0 Int)))
(rule (ack m_1 0) (ack (- m_1 1) 1) :guard (> m_1 0) :vars ((m_1 Int)))
(rule (ack m_2 n_3) (ack (- m_2 1) (ack m_2 (- n_3 1))) :guard (and (> m_2 0) (> n_3 0)) :vars ((m_2 Int) (n_3 Int)))
(rule (ack m_4 n_5) 0 :guard (or (< m_4 0) (< n_5 0)) :vars ((m_4 Int) (n_5 Int)))

Confluent by Parallel Closedness with proof:
no critical pairs

Elapsed Time:  17.85 ms