Silicone is highly permeable to oil. I don't think it would be a very good material for a CV boot as it would allow the base oil to leak over time. Also, many CV greases are silicone based which can cause deterioration of the silicone rubber boot. A silicone based grease and EPDM rubber boot would provide best longevity provided there is no contact with engine oil. The grease needs to be highly shear stable as there is no stirring action such as seen in a bearing to mix the oil and thickener.
Silicone boots look like a great application for show cars and showing off - lots of colors and so forth. They don't look like a good option for a vehicle that is driven a lot or is expected to last a long time.
To directly answer the question, those silicone rubber boots are probably the same material as oven mitts.
If interested in performance, rather than looks, real CV joint boots are made from materials specifically developed for the application by small companies with names like DuPont and Celanese.
