Do they even playtest recommended builds, or just randomly put them in?

Specifically in the case of Xayah. It recommends Rapid Firecannon as her Zeal item. Rapid Firecannon does not synergize with her kit at all due to her passive having a fixed distance.

Not only is it the worst Zeal item by raw stats, deals less damage total than Shiv, but the supposed synergy it might have with her kit doesn't actually exist. If it were to also extend the reach of the first passive feather thrown with an auto attack, then it would make sense. But as-is, that first feather actually falls short of the next ones thrown by the same distance that auto was extended, making it less likely that that feather will hit the retreating opponent on return.

Runaan's Hurricane, on the other hand, not only has better raw stats than RFC, but applies her W to the enemies hit by the bolt. It may not do anything especially unique in regards to her passive, but neither does any other zeal item. But if you aren't in a good situation to have that much uncontrollable waveclear, well there's Phantom Dancer. It has a whopping 15% more AS than RFC, 7% more movement speed if she's in range to AA-- since her range is 525 against the 550 requirement, a great dueling defensive passive, and costs 50 less gold.

Rapid Firecannon should not be the recommended item for anyone in the game, let alone a champion who synergizes much better with any other Zeal item.