Author here. This doesn't give the correct results. It produces meal_items that have both customer_id and employee_id. Here's an excerpt (the full result set is thousands of rows, as opposed to the expected 45):

    id |label       |price|employee_id|customer_id|
    ---|------------|-----|-----------|-----------|
    ...
    344|Poke        | 4.18|       3772|      13204|
    344|Poke        | 4.18|       3313|      13204|
    344|Poke        | 4.18|       2320|      13204|
    344|Poke        | 4.18|        632|      13204|
    344|Poke        | 4.18|       4264|      13204|
    344|Poke        | 4.18|        699|      13204|
    344|Poke        | 4.18|       1070|      13204|
    344|Poke        | 4.18|       3022|      13204|
    344|Poke        | 4.18|       1501|      13204|
    344|Poke        | 4.18|        808|      13204|
    344|Poke        | 4.18|       2793|      13204|
    344|Poke        | 4.18|       1660|      13204|
    344|Poke        | 4.18|        932|      13204|
    ...

To be clear, there are ways to write this query without UNION that have both good performance and give the correct results, but they're very fiddly and harder to reason about that just writing the two comparatively simple queries and then mashing the results together.