And just in case you're tempted to take it as factual because the "Gerald of Wales" guy was probably from the same period, you should remember that only a century or so later explorers were making the totally factual claims of fearsome cannibals with their heads in their torsos, or real live unicorns.
One thing that a longbow could reliably do was kill or startle lightly armored horses, or wear down an armored man at long range, but not impale his thigh in a single go.
That said, even the best plate armour has joints or other weak spots, and plenty of knights had to make do with kit bought secondhand or from the cheapest bidder. If the archers could put enough shafts in the air fast enough they were bound to get lucky sooner or later.
Oh sure, but you're going to have trouble hitting anything important at range and you're in a bit of a hassle if they got close enough. Weak spots in armours tended to be on the joints and such so even if you reliably hit those, it doesn't necessarily mean the bastard is out of the fight. Especially not when rondels came in to use. Those were essentially tiny shields held in place over these weak spots. And later plate didn't even bother with these either as advances in metallurgy and armour-smithing meant that they could create plates that curved and overlapped over these weak points.
You know how DF crossbows are killer even with wood bolts because goblins rarely armor several of their body?
Same idea.
Longbows had (with sufficient arc) a very long range, and high rate of fire, so enough archers could put enough arrows into the sky so that enough wounds were scored on the knights and their horses to immobilize (joints were one of the weak spots, remember?), demoralize, or...bleedize? the enemy. And, of course, longbows came into play at a time when there were a large number of peasants. There was a law in...I forget what century, but in Medieval England that every man had to practice longbow at least once a week. Even with the details that surely have either slipped my mind or were not mentioned, that's a hell of a lot of potential soldiers.
Used correctly, you could probably use longbowmen to absolutely destroy any other fighting force from before firearms overcame their initial downsides. (Of course, used
incorrectly, they would be the destroyed, but that's the case even with tanks vs. clubs if your tactics are weak enough.)