Yeah, agreed.
Most importantly, though, Abengane didn't say "It's impossible to be that damn good at Emission, Manipulation and Materialization! He must be a God or something!" he said "That guy has a level of Nen that allows him to use in a sophisticated way Emission, Manipulation and Materialization. That added to his physical agility... even if we all were to go after him, we would be taken down by that "Little Flower" of his." That was all to show the reader how good, skilled and outright dangerous Genthru was. In hindsight, anyone would have simply thought "...wait. He was not that good! Those 2 companions of his helped too! Haha!"
Also, they had deduced he had companions. If it was so easy to have a "shared ability" like that, Abengane (or someone) surely would have mentioned something along the lines of "It's probably it's, on the very least, a 3 man group. An Emission user, a Manipulation user, and Genthru himself being the Materialization user, proved by the fact that he was the one who clearly materialized the bombs." , but nothing of the sort was even hinted at. Actually, never was someone, somehow, capable of somehow influencing someone else's ability simply by "existing". Does it mean that if Sabu or Bara had been killed the ability simply "wouldn't work", because Sabu or Bara's Nen was adding to Genthru's ability because they "exist"? What if Sabu or Bara were sleeping, or KO'ed, or out of aura? It just... doesn't add up.
And on the Emission/Materialization issue, let's take a different approach. What if a Materialization user tried to materialize a clone? The clone would HAVE to be better in some way. Wing didn't mention that "materializing creatures is stupid." or anything like that, so it's supposedly a valid idea. Would the Materialiation user be able to create a "better" clone, that's stuck to his body?
Applying that in a more general case, it would mean that materializing creatures in general is the worst idea ever, and nothing of the sort was indicated or hinted at. Either keeping materialized creatures around requires a very weak Emission (doesn't make much sense to me), or it doesn't require Emission at all (this is what I think is the case). Emphasis on creatures. Obviously mantaining materialized objects away from you requires Emission, that was stated multiple times.
On a somewhat unrelated note, I also don't think Gido used any Emission to keep his tops spinning. IMO, that was solely Manipulation. For the same reason I think Kastro's clone didn't require Emission: it wasn't mentioned. Wing mentioned Kastro's attack (forgot the name) was very strong, proving he was an Enhancer. If he used Emission to keep the clone, Wing could have said "That's further reinforced by the fact that Kastro was also quite skilled at Emission to keep his clone existing, and, since it's next to Enhancement, it's clear he is an Enhancer."
Same thing for Gido: "The same is the case for Gido: His Emission was quite good to be able to keep his aura at the range of so many tops, even if they were small. That also serves to prove he's definitely an Enhancer." After all, Gido could not even have been an Enhancer: for all we know, he could have been manipulating his body's muscles/bones to make it easier to spin or something, like the guy from the mafia who manipulates hair. It would also serve well for the reader and Gon/Killua, who were noobs: the more examples of various types there was, the better it would be to help them (and us) understanding. Incidentally, Emission (alongside Transformation and Specialization) was one of the few types not mentioned, so it would have helped Gon and Killua to understand Hatsu better to say when it was used if it really was the case.