Time to take a step back. You have:

BuildDates.BuildEventType IN (dbo.DelimitedSplit8K(@BuildEventTypes,','));

This is the same as

BuildDates.BuildEventType = dbo.DelimitedSplit8K(@BuildEventTypes,',');

But since DelimtedSplit8K is a table-valued function, this is not going to work out. You need to query the table-valued function:

BuildDates.BuildEventType IN (SELECT col FROM dbo.DelimitedSplit8K(@BuildEventTypes,','));

You need to replace "col" with the actual column name used by the function (which I don't know by heart).