If the number of job openings go up, but at the same time the average job opening has worse pay and conditions, I don't see the connection to the lump of labour fallacy.

As far as I know, that's how the lump of labour fallacy actually works! There's an equilibrium between supply and demand for labour such that workers are (more or less) choosing the best jobs they can find. There's infinite demand for labour, but not at any price, so demand destruction of the best jobs can still happen. There will be infinitely more jobs, but they won't be as good as the ones that were destroyed.

Of course, demand creation for really good jobs can also happen. What the GP is saying is that they believe demand destruction will outpace demand creation for really good jobs and everyone will be forced into less desirable jobs. I don't see how this is inconsistent with the lump of labour fallacy.