That's more of a double-edged sword hack than "using DRM". The theory of DRM is for the system to restrict the content from the user, i.e. the system is adversarial to the user and vice versa.

What Signal is doing is trying to get the system to restrict the content from the rest of the system. Which might work as a transient hack but doesn't actually work to protect the user when the system is adversarial, because Microsoft (the adversary) has the DRM private keys. Even some hypothetical DRM system which is effective in oppressing the user wouldn't prevent Microsoft from purloining the user's data whenever they want because they're the ones who make the DRM.

"Used" is a strong term, they're not really utilizing the DRM codepaths, AFAIK it simply tells the OS that the window is software that does use DRM and thus should be excluded from any screenshots. The existence of DRM and desire of Windows to abide by its rules are what Signal relies on.