It's not. A bill of attainder declares a specific person guilty for past actions and punishing them - thereby denying them the right to a judicial trial.
While this bill does refer to a specific organization, it doesn't punish them for past actions, but rather constrains US companies in the future. That is perfectly ok as shown in cases like Huawei v. United States or Kaspersky Lab, Inc. v. DHS. Like Kaspersky, it's "prophylactic, not punitive."
The constitution makes no reference to citizens with regards to what congress can and cannot do. Nearly all these things are couched with "congress shall pass no law" -- the focus is restricting the behavior of congress rather than the citizenry.
This is a totally bunk argument. If you want an argument that does work against the constitution, SCOTUS' "empty shell" or Lysander Spooner's "Constitution of no authority" are of far more weight, but have more disturbing implications (e.g. You can disregard it completely; our system is precisely the "odious arbitrariness" the founders denounced).
While this bill does refer to a specific organization, it doesn't punish them for past actions, but rather constrains US companies in the future. That is perfectly ok as shown in cases like Huawei v. United States or Kaspersky Lab, Inc. v. DHS. Like Kaspersky, it's "prophylactic, not punitive."