The way the values of YAML variables are rendered in HA is through templates. Anything in HA that includes double-curly bracket delimiters, {{ }}, is a Jinja template.
You can use templates in triggers that support templates and only where they are supported. State triggers only support templates in the for configuration variable.
Community Cookbook - What the Heck is a Template…? - Where Should I Use Templates?
That blueprint does not use templates in State triggers… its trigger section is composed exclusively of MQTT and Event triggers.