My understanding is that R&A and USGA rules are now synchronized.
Rule 11 talks about the teeing ground:
When a player is putting a ball into play from the teeing ground, it must be played from within the teeing ground and from the surface of the ground or from a conforming tee (see Appendix IV) in or on the surface of the ground.
Note the reference to a tee "on the surface of the ground".
Appendix 4 defines a conforming 'tee':
A tee is a device designed to raise the ball off the ground. A tee must not:
• be longer than 4 inches (101.6 mm);
• be designed or manufactured in such a way that it could indicate line of play;
• unduly influence the movement of the ball; or
• otherwise assist the player in making a stroke or in his play.
Rule 11 seems to permit the kind of rubber tee being referred to and nothing in Appendix 4 appears to prohibit. Others may know better.