Yeah. It is. The mana sustain on top of the AOE spellblade proc is really good for jungling. It allows for even faster clears using less spells, and it keeps you in the jungle even when you don't have blue buff.
With Magus, you had to recall more often at lower item breakpoints so you could be effective as a jungler. You would also need to grab Catalyst or Tear as your second item to help you with mana sustain (or you could horde blue buffs all game). Those two items do not provide a lot of combat stats off the bat and are only strong items much later in the game in RoA and AA staff.
Compare that with Runeglaive; your second item can be something with meaningful combat stats such as
or
without the running out of mana inside the jungle. In turn, you end up with more AP along with other meaningful stats, passives and/or actives. Your ganking potential is more potent due to the increased combat stats and a spellblade proc; you also have increased mana to pull off ganks effectively. Staying in the jungle, potential to gank more often, and gold going to more effective items makes for a stronger jungling item.
Now. Everybody wants to point out that you cannot build Sheen items in conjunction with Runeglaive, but really, would you be able to build Lichbane easily with Magus? You have a few items that you would probably want before Lichbane with Magus: Magus enchant was first, then you would have to buy RoA or Tear into AA staff. After that you probably want Deathcap, Zhonya's, Abyssal, or Void staff as your next few items because they either give you more damage, more defenses, or more utility than Lichbane. Likely, you wouldn't be able to buy lichbane until your 4th or 5th item because other items are higher priority.
Having Runeglaive allows for a more open build path for junglers. It makes it easier for AP junglers to acquire more gold, and this makes for a more potent AP jungler.
Again, I do think it needs a little extra AP to make it a stronger item for champions who cannot use the spellblade proc very effectively. But it doesn't really need as much AP as Magus because it's a stronger item in the early and mid game already.