Sounded as if ABS was engaging

Are you sure the ABS was engaging or was the clutch slipping really bad to cause a vibration and shaking that feels like an ABS?

Clearly if you are dumping the clutch and the brakes are not yet applied then it is unlikely to be an ABS event.

https://www.2carpros.com/articles/clutch-symptoms

https://www.2carpros.com/articles/inner-workings-of-a-car-clutch

If this does not happen when hitting the brakes in normal conditions then again, I doubt the ABS is activating.

Does the clutch grind in any gears or hard to engage? If so, this is a clutch issue.

https://www.2carpros.com/articles/how-to-replace-a-clutch

Please review this material and let me know if you have questions. Thanks