The fact that Intel is offering to manufacture ARM cores for their custom foundry customers is not new. In fact, there are some Altera FPGAs with embedded ARM cores being manufactured by Intel already. The important thing about this deal is that ARM limited will now provide Hard IP for Intel's process technology.
To understand the importance of this, you have to understand a little more about silicon design and manufacturing than the average Slashdotter. Suppose you are some random fab-less chip designer that builds semi-customized ARM SoCs, a company like Rockchip or Mediatek for example. Generally the way you put together your new SoC is you buy a license for the ARM CPU design, then you buy a license for a GPU design from someone else, then you license a USB controller... so on and so forth, until you have all the building blocks necessary to make your new chip. Then you plug them all together, simulate, fab, validate, and ship.
Those blocks come in two different forms, Hard IP and Soft IP. Soft IP is basically a netlist... its a big text file that lists every transistor in the design and the interconnections between every transistor in the design. Usually soft IP vendors will give you the RTL, which is a more human readable language like Verilog which you compile in to a netlist. Hard IP on the other hard, is more like a vector graphics drawing or a stencil. Hard IP lists every transistor, its x/y coordinates on the silicon, and the exact shape and route of the copper wires. The problem with hard IP is every silicon manufacturer uses different shapes and sizes for their transistors and connecting wires (this is called the process design for the foundry), so a given hard IP design can only be built by the foundry it was designed for.
There is a program called a synthesizer that takes the netlist from the soft IP and generates the layout for the hard IP given a bunch of input parameters that describe the target foundry's process design, rather incredible really. The problem is not every design is "fully synthesizable" for example anything involving high speed I/O or analog (aka the "PHY" layers for modern busses: PCIe, USB, eMMC, Ethernet, SATA etc.) In any case, the pieces of the design that can not be synthesized need to be drawn by hand (aka human hands) using CAD software. For things like CPUs, usually there are some critical pieces that are drawn by hand, because a good human engineer can design a better, more efficient layout than the synthesizer can, at much greater expense of course. So depending on what percentage of your design is not synthesized, switching from one foundry to another can turn out to be a lot of work! This is the important thing here, ARM is providing ready to go hard IP for Intel foundry, just like they do with TSMC already, so the technical barrier for an ARM SoC designer to use Intel foundry is now lower... potentially comparable to TSMC.
Depending on the amount of engineers you have and how sophisticated they are, you might design some of those blocks yourself. Up to the point of companies like Apple and Qualcomm where even the ARM CPU design is a custom implementation and doesn't bear much resemblance to the reference design from ARM limited.
For Intel, using Intel foundry is a non-issue since they have an army of engineers that for the most part they design every IP block themselves anyway. For companies like Apple and Qualcomm that also have armies of engineers switching to Intel foundry is not a technical issue, its about business decisions for them. The big news is the smaller companies that don't have as many resources to do custom design now have Intel foundry as a viable option.