Is it true that Sheikh Ibn 'Uthaymin issued a fatwa that a person who abandons prayer but prays Jumu'ah is not a disbeliever, relying on the basis that the Messenger's saying "prayer" includes those who abandon it entirely?

We have not found in the fatwas of Sheikh Ibn 'Uthaymeen or his treatise on the one who abandons prayer any fatwa regarding this question, and it is possible that this was heard by word of mouth. You can refer to fatwas 37472, 37735, and 38392 concerning the ruling on the one who abandons prayer.