The reference to Arsenal is just lightheartedly suggesting that his dribbling skills (in the football sense) are good enough for him to play for a top football/soccer club. When a player is "capped" it means that they have played a test for their country. This old tradition varies between countries but there are usually actual caps involved. The Welsh cap video refers to the fact that a player might be selected for the side that day as an emergency, but will not be officially recorded as having played a test match unless he actually takes the field. The game was already in injury time when the number 8 went off, so the replacement had to quickly get onto the field before time ran out to get his first "cap". In those days the replacements often sat in the front rows of the stand. He just made it, getting rid of his chewing gum on the way.

6:15 Substitutions are a relatively new thing in rugby. The first few years of my 'carrier' they were not allowed. At the time of this clip the rule was that you were allowed 2 subs and an MD had to declare that the one going off was unable to continue play. Everyone that started a match was expected to finish it. You had to pace yourself. No one could play 'hard' for 40 minutes, burn themselves out and then go take a break while a fresh body comes on to 'finish'. The current Laws are dangerous to players not being subbed out. Think of it like boxing. If, after 10 rounds, one corner replaces their boxer with a fresh fighter to take on the worn down and bruised opponent.