Intel bought FPGA maker Altera a while back. That is how they acquired an FPGA line of products. Some Altera Stratix 10 do have ARM cores in them.

ARM don't make chips. They design them. Then they license the designs to chip users, who in turn get chip manufacturers to make them. Intel make lots of ARM chips (as well as their own in-house designs).

Yes, absolutely. AFAICT this is an Intel-branded design from the Altera acquisition. Pairing an FPGA with an ARM host CPU is a reasonably common thing.

The Stratix 10 SX has an onboard AArch64 ARM core, correct. Previous systems used ARMv7. Most FPGAs with a built-in system-on-chip you'll see have ARM cores attached, as well, such as Xilinx boards. Intel isn't unfamiliar with selling ARM chips, but in this case -- Intel acquired Altera, who did this originally. You can find one-or-two FPGA SoCs from Intel with actual x86 chips instead, though.

But the idea is that the ARM cores normally just act as your "control plane" -- manage the FPGA, watch power usage, reload configurations, monitor & reporting metrics, etc etc. So they aren't the main attraction, of course.

In the long run, Intel will be moving to discrete PCIe FPGA cards for data center usage, as well as fusing the FPGAs directly on-chip, alongside the Xeon cores for extremely high-end stuff. That's where you'll find products like Stratix 10 (aside from a few other places.) So in datacenter-oriented offerings, you may not see the ARM cores floating around that much.

They did so for quite a while but somehow didn't really take off.

As I understood intel has the best FABs. About half the advantage over AMD lies purely in the production technology. ARM is said to be more energy efficient than x86. Yet, when intel makes ARM chips you don't hear them outperforming everything else. I wonder why.

It may be a surprise for you, but intel once had bridge chips with multiple arm cores working as audio codecs and ATA controllers back a decade ago.