Alternative to a silicone mold
You could just as well print the pot twice in different sizes and use those prints as a mold. You'll probably need to print both sizes at the same time because you cannot fit a slightly smaller pot through the opening of the large pot. You can print them with thin walls because they only have to withstand some liquid concrete, but no wear and tear.
Take a square wood or a board and glue the upper edge of the smaller pot to it. Then center the bigger pot over it and glue the edge. You now have a mold you can fill with concrete.
Once the concrete is completely cured, you'll have to break away the outer 3D print. You can chose to keep the inner print as a water barrier or break it away as well for a pure concrete look.
This flower pot has a very complex surface. That's what makes it so attractive for 3D printing and the reason why such objects were very rare before 3D printing: it cannot be mass-manufactured in the conventional way.
The rough shape is a cylinder with some very prominent protrusions. If you cast the outside of the pot in a single mold, it will be very hard getting the pot back out after the mold cured. If you also cast the inner space as a single mold, it will be almost impossible to get the concrete cast out of there without either destroying the mold or the cast.
You need to cut the outside mold vertically into at least 2 parts, but 3 are probably better. Due to the irregular surface, it will be hard to create clean lines. It's better to seal the opening of the pot, lay the entire mold container onto one side and pour molding silicone up to 1/3 of the volume. Put some big indentations into the silicone (with something like wooden dowels) to help you align the mold parts.
Once the first part is completely cured, remove the dowels (but not the printed pot!) and cover the whole mold with a very generous amount of mold release. Rotate the mold by 90° and pour a second part of the mold like above (including indentations for alignment). Once that's cured, stand the container upright, add more mold release and pour the third part.
You'll encounter the same problem in the cavity of the pot: fill it with silicone and you'll have a really hard time getting it out in one piece again.
I propose finding (or printing) any cylindrical object that is almost as big as the cavity at its smallest part. When you fill the cavity with molding silicone, suspend the cylinder in a way that it fills as much of the cavity as possible without actually touching the walls. Once the mold is completely cured, you can first remove the cylinder, which should give the silicone enough room to be removed as well.
Keep in mind that the cylinder is part of the mold and needs to be inserted when you pour the concrete.
Concrete is thick and heavy. If you don't want your pour to distend or leak, you need either very thick silicone or a structure that gives the silicone more stability. You should not "break away" the molding container at all. Instead, pull the cured mold out, separate the parts and remove the original printed pot.
When you want to pour the concrete, reassemble the mold parts and insert them into the container to give them stability.
Why would you "pack some clay on the inside so that the resulting wall thickness is sufficient for concrete" when you could 3D print the pot with a sufficient wall thickness? At the very least, I would greatly increase the thickness of the bottom of the pot.
If you want to save on printing material, a better solution might be filling the entire inside of the pot with clay except a smooth cylinder (very similar to the one I suggested for making the inside mold). That makes the molding process much easier, but it might create weaknesses in the wall.
The finished concrete pot would have the structured outer wall but a simple, smooth cylindrical inside.
You'll need a 2 part molding silicone for this project and it should be pourable. Be careful when mixing both parts to avoid introducing air bubbles. Pour the liquid silicone in a thin stream into the mold (that removes bigger air bubbles) and don't pour directly over the object. Instead, start pouring at a side wall or the bottom of the mold container and let the silicone rise up around the object to avoid trapping air bubbles.
You'll want a rather liquid concrete mixture to make sure that all the protrusions are completely filled. Shake, tap or vibrate the filled mold to help air bubbles escape.