What the TikTok watermark actually is
When you save a video from inside the TikTok app, TikTok makes a fresh copy with two things burned into the picture: the animated TikTok logo that bounces between corners, and the creator's @username. Because they are baked into the frames themselves, no cropping or filter can cleanly remove them afterwards without cutting into the content.
IgAnony avoids the problem instead of trying to undo it. Rather than re-recording the exported copy, it saves the original video — the same file the app plays when the video comes up in your feed. That original was never watermarked, so what you download is clean to begin with, not cleaned up afterwards.