TL;DR:
(in front because this comment is very possibly not worth reading in full)
Morello and Void might be roughly equal at 90 MR if you consider total gold efficiency on both sides, but it would vary wildly from one game to the next. In most cases Morello would be more efficient so u/a_lol_cat is basically correct anyway. I just felt like going into more detail for no reason
I think he must have been talking about gold value, and also mostly about a full Morellonomicon. Once we're talking about gold value you have to start considering the value of the health that comes from Morello's. I'll leave out the Grievous Wounds passive in comparing the two but it's worth keeping in mind.
Morello has the same AP as Void, and although it costs 350g more, it has 801g worth of HP. You also left out Sorcerer's Shoes in your calculations. At 90 MR, Sorcs+Morello gets it down to 57. Sorcs+Void gets it down to 43.2, which is still about 13.8 less MR you're dealing with if you have Void Staff. So you're still correct in terms of raw damage.
Now we get a little bit into the weeds about gold efficiency overall, not just in a vacuum. Everything from this point is theoretical because it depends on everything else in the game... but in an average game Kobe may have been right. Maybe.
Consider that the opponent has to have 13.8 more MR in order for Void Staff to do more in terms of magic pen over Morello. Not accounting for completed MR items, that much raw MR amounts to just under 250g. Meanwhile, if you consider the gold value of Morello's HP being 801, minus the 350g difference between it and Void, you have an extra 451g in raw stats, and if the opponent has 90 MR, you end up having 200g more in stats than they do - but again only in a vacuum; with completed MR items it would probably be close to equal.
Again that last paragraph depends so much on the context of each individual game. I don't think Kobe was using 90 MR as a proven figure, but more as a theoretical baseline for gold efficiency. Depending on the game it can vary wildly and in most cases Morello would still likely be the most efficient.