"we want to broaden the number of champions that can tap into an AP jungle item"
we want to broaden (you might say diversify) the number of champions that can tap into an AP jungle item. The main problem with Magus was that it didn't make champions functionally better at jungling (like say, Cinderhulk); instead, it gave champions who already jungled well a pretty large stat infusion in the midgame. What you end up with is the same effect observed with Luden's Echo or top lane Challenging Smite - a narrowed champion pool defined by who can, or cannot, use the item well.
That last sentence... is EXACTLY what Runeglaive does.
How can you say that Magus narrowed the champion pool depending on how well they use the item? Yeah, there are so few AP champs that can make use of a bunch of AP and CDR. Who the hell needs those stats? Runeglaive on the other hand has mana and a spellblade, both things that EVERYONE uses right? RIGHT?!?
While I don't entirely disagree with the change (fiddlesticks was OP and this'll give a chance other AP 'junglers', basically because of what they said before (the part that does make sense): "it gave champions who already jungled well a pretty large stat infusion in the midgame") I think that last sentence of the paragraph was very inapropriate. Magus gave way more universal stats, the only problem was that it didn't help as much in the jungle. You will definitely see some AP junglers rising, but right now the AP junglers that will rise are a narrowed champion pool defined by who can, or cannot, use the item well.
Couldn't be happier.
etc now when I play them as Jungle (Yes Lulu and Zyra jungle work, and they actually work very well for some reason.)

as its component. AP playstyles in general are defined by spell usage, and only few AP champions would feel comfortable using their autoattacks while clearing the jungle or spacing out their bursty combos to make the most out of the item. Fewer still would find the item significantly useful out of the jungle, and as you mentioned its Spellblade effect is pretty weak. I also think the item makes no effort to reach out to potentially interesting AP picks that currently struggle to clear the jungle, especially since the change wasn't accompanied by any other improvements to AP jungling. I think it would be good to keep the mana sustain and clear enhancement on the item, but move those effects instead to something more spell-focused, which would appeal universally to all AP junglers in the same way
appealed to all tanky junglers.