Updating array formula from excel 2019 to use LET in excel 2021

Hi All, a couple of months back I was given help on the attached workbook, which by using array formulas solved the problem I faced. At the time I was using excel 2019 and was told by several people on here I should update to excel 2021 as things would be a lot simpler and no need to use array formulas. At the time HansDouwe gave me two formula for cells O5 & O6 on page "Records" which now I have excel 2021 I have added to the workbook and they work just fine. I'd now like to complete the grid using these formula but I just don't understand them.

For example HansDouwe suggested this

=LET(t,FILTER(ALL!L2:L9999,ALL!E2:E9999="H"),MAX(FREQUENCY(IF(t<>0,SEQUENCE(ROWS(t))),IF(t=0,SEQUENCE(ROWS(t))))))

I tried to adapt this for O4 by removing the part ALL!E2:E9999="H" which was no longer needed so

=LET(t,FILTER(ALL!L2:L9999),MAX(FREQUENCY(IF(t<>0,SEQUENCE(ROWS(t))),IF(t=0,SEQUENCE(ROWS(t))))))

but I get the response I haven't entered enough arguments for this formula.

If somebody could break down the formula for me and tell me what it is actually asking that would be great. As always any help much appreciated.