Odd: In your last post he was punching the bag. Now he's kicking it. Never mind that how hard Bruce Lee kicks has nothing to do with the point you were trying to make about how hard he could punch, maybe you could make up your mind: which is it?
In any case, you're basing this, I assume, on that single quotation on the website you made reference too. Well, here's the thing: we have an eye-witness account, given possibly several years after that fact, which provides few details. Never mind that, in hindsight, this fellow might very well be slipping into hyperbole. In actual footage of Lee training, which I unfortunately cannot provide here, you can see him kicking, for one, a pad held by another man, and then what appears to be the infamous heavy bag on his patio, as described by Mr. Jay described it. My first observation: the bag looks too small to be 300 pounds (perhaps Lee had more than one bag on his patio? Who knows?). My second: the footage consists of Lee going off-screen then running back to deliver a side kick. This is how he kicks, again and again. The bag certainly goes quite a ways but it damn well ought to when the guy is getting a running start before hitting it.