Why did 'Umar ibn al-Khaṭṭāb describe the Tarawih prayer as a "good innovation," even though his action is part of the Sunnah of the Rightly Guided Caliphs, which we are commanded to follow?

Umar ibn al-Khattab's (may Allah be pleased with him) saying, "What an excellent bid‘ah this is," regarding the Tarawih prayer, does not refer to the blameworthy, religiously forbidden bid‘ah. Rather, he intended the linguistic meaning of bid‘ah, i.e., a new matter that was not previously practiced regularly under one imam. This is despite the fact that the congregational performance of Tarawih prayer is originally sanctioned in the Prophet's Sunnah, and the Prophet (peace be upon him) abandoned it out of concern that it might become obligatory for the Ummah. This concern was removed with his passing.