See, this is where we get into the endless debates about GM-adjudicated things. You are correct, up to a point, IMHO. Beyond that... consider, a 10x10x10 block of stone weighs on the order of FIFTY TONS. It is going to the bottom of that pit, and your cube is going to barely present an obstacle, its going to be squirted out of their like nothing (I am going to assume the trap designer was smart enough to insure that the block is somewhat smaller than the pit's dimensions). Honestly, the big problem would be making sure the block falls cleanly into the pit and doesn't get stuck, etc. but that's an 'engineering' problem and could be plausibly solved.
Where are we get 10x10x10 for the dimensions of the falling block? None are specified AFAICT.
As
@Crimson Longinus notes, it's obviously not even remotely possible that it's a 10x10x10 block because it's held up by a single rope, which is not going to hold 50 tons. I read that a 1" rope of the type usually found in D&D can probably hold somewhere up to maybe 8000lbs on a very good day. You're proposing it holds up 112,000lbs, which is, what 14x that? So we can figure that, at most, the block is 1/14th of 10x10x10, so 71 cubic feet. So 4.1x4.1x4.1 maybe? Unless that's not how cube roots work. That's a hell of a lot smaller and 14x lighter - what is going to happen is it will displace some of the goo as it rapidly sinks.
You might claim that could cause splatter, but wait, we've established the pit is 11' deep so it can have the 1' lip described. So that means it has 100' cubic feet of potential displacement before anything goes out of the pit. Which is more than 71' cubic feet. Now the 4x4x4 block is hitting at maybe 6-7mph, so there might be SOME splatter, depending on how viscous the Gelatinous Cube is, but frankly, it can absorb a person walking or running into it at 4-6mph without problems, so I'm guessing even for a much heavier thing with more displacement, it's not going to splatter hardcore. And again, the shape of the pit dictates the spray goes UPWARDS, and then probably comes back down almost vertically. Depending on the exact physics some may go over the edge but won't be much.
You haven't addressed the other various failings of the situation, like the fire killing the Yellow Mold, either.
As for implausible, you're confused.
There's implausible as it unlikely to exist/be set up, which can be excused under certain circumstances or as a genre trope. That's fine, and then there's implausible as in "not believable to function as described". If this trap was implausibly high-tech involving hydraulic pistons or w/e, fine, genre trope. But this is just "dude in 1980 didn't understand basic everyday physics".
But let me revise my "implausible" to "impossible". The trap simply
could not function as described. It's so extremely bad, too, that many players will immediately start asking questions about it. If you use it to actually kill PCs, as is intended, those are likely to be quite searching questions and deserving of actual answers.