Wow, on a whim, I was looking at the core type settings for boolean.
The boolean type Maps to the JSON boolean type. It ends up storing within the index either T or F, with automatic translation to true and false respectively.
The answer is:
doc['top_seller'].value == 'T' ? 10 : 0
Edit: As of 5.2.x, I am finally able to use doc['top_seller'] ? 10 : 0
. https://www.elastic.co/guide/en/elasticsearch/reference/current/boolean.html