Background: HEC uses 3 pins. Those pins can be used for 3 different protocols: HEC (100Mbps differential signal slightly modified from the Ethernet standard)(1), ARC (1 Mpbs SPDIF signal over a single wire)(2), and eARC (~40 Mbps SPDIF differential signal)(3).
Modern computers with integrated graphics send the HDMI signal from the CPU pins. But in all the examples I could find, those pins are all Displayport (4)(5)! This is because DisplayPort supports HDMI video data wthin its signal. This allows manufacturers to convert that to HDMI with a cheap IC (that is mostly a redriver and level shifter). So they just have a DisplayPort signal exit the CPU/GPU and convert it to HDMI if necessary.
At this point I should note that DisplayPort does not support high-bandwidth auxiliary channels. So higher bandwidth features like HEC would require the converter IC to have a separate data channel to the CPU/GPU. But I couldn't find a DP-to-HDMI IC that supports HEC or eARC. I guess there's just not enough interest.
In theory, some computer/GPU manufacturer could connect the HEC pins to an FPGA that has a high-bandwidth connection to the CPU/GPU to support eARC. But even with that, your hacker would need to reprogram the FPGA to stop being a SPDIF encoder and start being an ethernet bridge. And this hypothetical FPGA can't support just ARC - it would be way too slow to be reprogrammed for Ethernet.
Of course, this leaves out home theater devices. They support eARC, and if their ICs are flexible enough, might be able to speak Ethernet with a lot of programming. But since no devices support HEC, you would have to reprogram devices on both sides of the HDMI cable. And at that point, you could just transmit data over SPDIF.
In short, there's a 99.9% chance that HEC pins are physically disconnected in any computer's HDMI port. Even if they were connected, a hacker wouldn't be able to establish an Ethernet connection unless the device under attack is using a very obscure IC or is ridiculously over-engineered.
1: https://patents.google.com/patent/US20110096793
2: https://www.ap.com/technical-library/more-about-arc/
3 (PDF warning): https://www.latticesemi.com/-/media/LatticeSemi/Documents/Wh...
4: https://www.reddit.com/r/Amd/comments/gzkxux/the_am4_pinout_...
5: https://images.anandtech.com/doci/14657/ASUS%20Pro%20WS%20X5...