The ACLU tends to take pretty hard line civil liberty positions, including defending hateful folks if their civil liberties are impinged. They’re not strictly a progressive organization.

Classic "Bootleggers and Baptists" situation [0], where both parties are in favor for self-serving reasons.

[0] https://en.wikipedia.org/wiki/Bootleggers_and_Baptists

What does the ACLU not understand..

The law does not ban TikTok.. it requires divestment from a foreign adversary..

Said foreign adversary refuses to divest, thus the company is shutting itself down