I think you can accomplish this with a join to a copy of the data source.
(In Excel vernacular, the lookup range is a 2nd table, so you just have to create such a table in Tableau.)
This is accomplish in Tableau by bringing the data source in again and then going to the Data menu, Edit Relationships, and then setting the "Key Current" calculated field = to the "Key Other" field from the second data source.
However, I am unsure how your lookup function in Excel is determining which Keys to link, because the key value does not provide enough specificity to identify a single row. The "Other Estimate" column is checking the "Key Other" field value against the "Key Current" column, but depending on how the data is sorted, even with your "FALSE" argument, Excel will complete the lookup and result with the first exact match it finds in the array.
My guess is that you also want to match on the App ID.
I have attached what I came up with so far.
Thank you for that suggestion. It worked, but it seems strange that I have to use this technique to do what I want. I guess it's about the data structure.