YES

(format LCTRS :logic QF_LIA)
(fun O 0 :sort (Unit))
(fun ack 2 :sort (Unit Unit Unit))
(fun s 1 :sort (Unit Unit))

(rule (ack (s m_0) (s n_1)) (ack m_0 (ack (s m_0) n_1)) :vars ((m_0 Unit) (n_1 Unit)))
(rule (ack (s m_2) O) (ack m_2 (s O)) :vars ((m_2 Unit)))
(rule (ack O n_3) (s n_3) :vars ((n_3 Unit)))

Confluent by Parallel Closedness with proof:
no critical pairs

Elapsed Time:  12.92 ms