NETWORKDAYS — no one-liner in T-SQL

Business days are a table, not a function. Bank holidays live in that table too.

SELECT COUNT(*)
FROM Calendar
WHERE day >= @start AND day < DATEADD(day, 1, @end)
  AND is_weekday = 1
  AND is_holiday = 0;

DATEDIFF(day) counts Saturday. That is not NETWORKDAYS.

Canonical translation

SELECT COUNT(*)
FROM Calendar
WHERE day >= @start AND day < DATEADD(day, 1, @end)
  AND is_weekday = 1
  AND is_holiday = 0;