[SalesForce] Formula field for the Difference between 2 time values

You are correct - Looking at the SFDC Object reference, Case.closedDate is a DateTime field so line 2 and line 4 will break

Looking at line 2 and line 4 - these are manipulating days, not seconds so I'd rewrite line 2 and line 4 as:

Line 2

( 5 * FLOOR( ( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8) ) / 7) +

Line 4

MOD( DATEVALUE(ClosedDate) - DATE( 1900, 1, 8), 7) +

You should go to the doc and use the feedback form to point this out to SFDC

The examples which are mentioned in the documentation contains the following:

Finding the Elapsed Time Between Date/Times

To find the difference between two Date values, subtract one from the other like so: date_1 — date_2 to return the difference in days.

Finding the elapsed time between two Date/Time values is slightly more complex. For simplicity, the custom field Diff_c refers to the difference between two Date/Time values. Since Diff_c is a number, you can use this formula to convert it to days, hours, and minutes.

IF(
  Diff_c > 0 ,
  TEXT( FLOOR( Diff_c ) ) & " days "
  & TEXT( FLOOR( MOD( Diff_c * 24, 24 ) ) ) & " hours "
  & TEXT( ROUND( MOD( Diff_c * 24 * 60, 60 ), 0 ) ) & " minutes",
  ""
)