I don't know about rice bags but I do know that it's ill advised to bare knuckle punch anything that is dense and solid (e.g a boxing bag) for too long because your knuckles will eventually start to wear down and cause problems with finger movement, and I've seen it first hand, being a guitarist I avoid this at all cost. Doing it for 5-10 minutes without going flat out is definitely good and something that I'd advise, same with knuckle push ups on medium-solid surfaces such as wood flooring, but same deal as above. Avoid concrete with knuckle push ups unless it is a small amount. Also keep in mind that this kind of conditioning will undoubtedly affect you later in life, if you want an example of this then look at what happened to Mohammad Ali after taking that many hits to the head, it seems stupid but it applies directly to this too.

he said it's better than punching hard object to strengthen your knuckles cuz that's a quicker way to get hurt and possibly break your knuckles or wrist if done incorrectly

That kind of conditioning won't help if you do not punch correctly, not to mention that as you build up punching power, good solid pads will condition your knuckles for you. It also does not condition the other bones in your hand that will aid in shock absorption same as your wrists, and a very good example of this is Mike Tyson breaking his wrist in a fight because his wrist could not handle the immense power of his punch thanks to using wrist straps and gloves in pretty much all of his training.

The main causes of injury are knuckles hitting bone and inadequate wrist support when punching. If you do not punch correctly and you flat out try to destroy the target you will injure yourself, even if you do 1000 knuckle push ups a day and can punch a brick wall without it hurting, your wrist will buckle and the rest of your hand will shatter under the force of your punch, just keep that in mind.