Okay so let's quote - verbatim - from Storm King's Thunder, an official adventure supplement straight from WotC:
Okay, so Sir Jordeth is conclusively not merely a
knight, he is specifically a
knight. And he is specifically mentioned as fighting two
Frost Giants to a qualified win (killing one giant, making the other flee, but losing his squire).
A
Knight has 52 HP, AC 18 and a +2 AC version of Parry (spend reaction to get +2 AC against one melee attack). It can either make two Greatsword attacks at +5 for 10 (2d6+3) damage, or one Heavy Crossbow attack at +2 for 5 (1d10) damage. He's a CR 3 creature.
A
Frost Giant has, wait for it, 138 HP. Its AC is 15. It can either make two Greataxe attacks at +9 for 25 (3d12+6) damage, or one Rock ranged attack at +9 for 28 (4d10+6) damage. He's a CR 8 creature.
Now, if we simplify the battle to one on one, no range, no horses, we see that the Knight hits the Giant on rolling a 10, so that's 2x10x55%=11 damage per round, which means it'll take a Knight 138/11 or more than 12 rounds on average to kill the Giant.
Reversely, the Giant hits the Knight on rolling a 9, so that's 2x25x60%=30 damage per round, which means it'll take a Giant two rounds to kill a Knight.
Apologies for drawing out the obvious conclusion, but clearly, the Knight is entirely the wrong stat block for representing somebody claiming to have survived two Frost Giants (even if we charitably say he faced them one at a time).
The characters might never face this
Knight Knight in combat. But as soon as they fight alongside him (the story does say Sir Jordeth asks the party to hunt down the remaining Frost Giant) they will immediately go WTF - how could he have defeated pretty much anything, let alone something as fearsome as a Frost Giant!
My point here isn't to bitch about how improbable this is - my point is that it would have been trivial to represent this particular
Knight with something other than the
Knight stat block.
Take the
Champion (from Volo's) as an illustrative example.
A
Champion has 143 HP and AC 18. It makes three attacks each round, either Greatsword at +9 for 12 (2d6+5) damage or Light Crossbow at +6 for 6 (1d8+2) damage. So far, the hit points look more in line with the mission, but nothing else is that much better. For a level 9 fighter, I guess 3x12 damage is okay, but the Champion has 22(!) hit dice to pull off even these not-really-stellar numbers.
But this NPC got a funky "class ability" I'm sure many players would
love to learn - as long as the Champion has more than half its hp remaining, it deals another 7 (2d6) damage
on each attack.
Now suddenly it doesn't look all that far-fetched to believe Sir Jordeth's tall tale.
A
Champion Knight hits the Giant on rolling a 6, so that's 3x19x75%=43 damage per round (at first). This kills off a Giant in just over three rounds.
While the Giant still hits the
Knight on rolling a 9 for 30 damage per round, so it needs almost five rounds to down the Knight.
The numbers still indicate Sir Jordeth had a large dose of luck, but then again, we didn't include the warhorse or the squire in the math. And we weren't there

. Perhaps the first giant targeted was the one to flee as soon as Sir Jordeth managed to put the hurt to it? Perhaps the second one didn't arrive for a crucial few rounds, and then wasted most of its efforts on killing the poor squire and/or missing a lot.
Anyway, the details of the math doesn't matter. What matters is that the
Knight is entirely implausible while the
Champion is perfectly credible or close enough that I won't make a fuss about it.
---
So why then insist on putting
Knights out there? "They're lazy" or "they can only use NPCs from the MM" are inadequate answers.
I guess my real point here is that WotC wants to
both eat and keep the cake.
But they can't both put the focus on player characters (by having every NPC be an insignificant low-level wimp) and
still portray NPCs as men that eat Frost Giants for lunch.
Either stop saying NPCs can do these things, or give them the stats they need to back up their claims.