Incorrectly identifies true as volatile

[tyre]

backend/generated_sql/upgrade.sql:2:2: warning: adding-field-with-default

   2 | -- Running upgrade b9faaaa7c3f1 -> eb171722ab54
   3 | 
   4 | ALTER TABLE payer ADD COLUMN allows_concurrent_billing BOOLEAN DEFAULT true NOT NULL;

  note: Adding a field with a VOLATILE DEFAULT requires a table rewrite with an ACCESS EXCLUSIVE lock. In Postgres versions 11+, non-VOLATILE DEFAULTs can be added without a rewrite.
  help: Add the field as nullable, then set a default, backfill, and remove nullabilty.

I believe this is incorrect. The default is the boolean true, which is about as non-volatile as it gets.

Am I missing something?