If it remains a joy today (which I am sure it does), then the native writer has the choice of verb tenses there. Standard 'reported-speech' exercises would expect the student to regress the verb to the past form, and that is indeed what native speakers generally do except in the case of 'present or universal truth', whose verb form is sometimes at the mercy of the speaker's image at the moment of utterance. He explained that water boils/boiled at 100°C.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.