TDH operation analysis of the distinct and groupby

TDH distinct in operation and groupby Analysis
calculated the same order of magnitude faster than distinct Why groupby

select count(1)
from
(
select cust_isn
from 
    database.table
group by 
    cust_isn
)

select count(distinct(cust_isn))
from
    database.table;
    

The operation takes 24s distinct, the operation takes 1s groupby

inserted here described image
groupby of the DAG
two stages shuffle
last node takes 1s

dinstinct of the DAG
a shuffle stage
the final stage takes 24 seconds

reasons Summary:
Although more than a shuffle operation groupby than dinstinct, but because there is a task groupby calculated expected, leading to faster groupby