##RAW, the ranger should have been able to pull the warlock only as they were pulling someone grappled

I think you're right about your finding :

 - The spell never states the target becomes weightless

 - The spell never states anything about somebody else moving the target

 - The spell "allows [the target] to move as if it were climbing.", so it still takes the target's movement into account [per the rules][1].

So, technically, for the ranger to pull the warlock, the only rule I can think of is the "move a grappled creature", where the creature can move half of its movement.

##But I think your first ruling was totally okay

Rule of cool totally applies here, I think your ruling was good. This was a great idea and you rolled with it. Still, like every time the rule of cool is invoked, I would warn the players to not try this too often. 

## Edit : Another rule I just remembered : The push, drag or lift.

Thanks to the comments under this answer, I realized that the Grapple *can* be use but doesn't really takes into account the fact that the target is willing. Then, I though : "The Warlock is already as fast as an anchor, what if we stop consider them as a creature and just as loot to transport ?". Here comes the "[Push, drag or lift rule][2]" :

> You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet.

This allow the floating warlock to be pushed around by another character, as long as they are strong enough !

  [1]: https://www.dndbeyond.com/sources/basic-rules/adventuring#ClimbingSwimmingandCrawling
  [2]: https://www.dndbeyond.com/sources/basic-rules/using-ability-scores