They're a 501(c)(4), not a 501(c)(3). They aren't nonpolitical and have no obligation not to engage in politics. In fact, engaging in politics is the entire point of the ACLU
"Political or social activities. The promotion of social welfare does not include direct or indirect participation or intervention in political campaigns on behalf of or in opposition to any candidate for public office."
https://www.law.cornell.edu/cfr/text/26/1.501(c)(4)-1
I think this is a fair confusion currently however given their strong dem tie-in recently and their current approach of taking on individual politicians (for and against) by name.
Note: If you are active in the ACLU they have a related ACLU Foundation they use to collect 501c3 money, is run by the same people out of the same offices, and actually brings in more money I think than the c4 if that matters.
That said, this is still a real shift for the ACLU - this type of campaigning is not something they historically really did. So the push to actively campaign more is newish for them.
It's def where the money is though - the MAGA orgs, the NRA, the ACLU - in some ways they are just following the money.