I’ve never heard anything like that before, it makes me think the violin is made of rubber. I can’t imagine anything other than wood being a good material for a violin, so whatever it is I would highly recommend they learn on a different instrument. The only type of material that I have come around on other than wood is carbon fiber, but that took me a while to feel enough confidence to say so. Very interesting!
Some of your comments regarding your thumb/technique make me think you have too much hand tension when you are doing your vibrato. There shouldn’t be any sort of grab on the fingerboard where your fingers (thumb) would have enough stress to double-joint. Would you mind posting a video of you here in the forums so I could take a closer look? You definitely don’t want your thumb to not touch the fingerboard as this will negatively effect your shifting and intonation. My guess is you are holding the instrument too much with your left hand (instead of chin and shoulder). Just a thought, would love to help!