Check this out

Actually MDF will not change with temp and humidity. I once soaked a piece of MDF in water for days and could not measure the change with a micrometer. I can't speak for acrylic as I am not sufficiently familiar with the characteristics but perhaps a materials engineer out there can answer this.
You ask how many turntables have MDF bases. I would say a great many starting with the rega. How about speakers as well. Most all are made with MDF and a wood veneer. How many solid hardwood speakers do you know of?

As far as subtle changes in the wood. I am sure that teres has accounted for the most significant changes in their design. I certainly would not expect movement observable to the unaided eye or I would quickly ship my table back. But as you yourself have pointed out to all of us many times in this forum, turntables are exquisitly sensitive devices where tremendous differences in sound quality can be achieved with very subtle changes.
So let's see, should I play my records on a base with random and uneven density that is subject to even subtle expansion and contraction? For my part, I think not. But the ear is king so if it sounds better to you, then I say go for it!

To each his own. I have no interest in making an argument over this. I don't even have the blasted table. And it is likely that I never will, because I can't afford it anyway.

They made the table that way because they thought it sounded better and felt they could make it stable. Time will tell.

One final response to Dan ed ....
I say wet wood because all wood is wet at some level. The cell structure of wood makes it like a sponge, even when kiln dried, the moisture content is only reduced, never eliminated and will still change to be in equilibrium with the surrounding air.

No TWL does not work for Teres but I clearly appreciate his support. Thanks Tom!

About PEG. PEG only works with wet or green woods and it is not effective with the dense woods we use. Yes kiln dried wood still has some moisture, but not enough to work with PEG.

I can't believe that you are seious about soaking MDF and it didn't change dimensions. MDF has terrible dimensional stability. It swells like a sponge when it gets wet. The advantage of MDF is that is does not warp. I have done a number of projects using painted MDF and you can always see the joints after a short time. It's no worse than solid wood but it certainly is no better. I have tried MDF for a turntable base and it's not bad, but nothing like hardwood.

Yes I realize it works with green (or water logged) wood. You would treat the wood while green then let it dry. I believe it has been used by woodworkers successfully on a wide variety of wood densities. Many turned bowls and vases are made from the exotics you use by just this method. You have to use PEG of the appropriate molecular density - but you have no doubt researched this more than I.
As for MDF, there are newer, moisture resistant varieties that are pretty stable compared to wood when soaked - at least in my experience. I believe they use phenolic resins. But of course, they still will swell some since they are, after all, a wood fiber product