HBM2 is totally tame in terms of signalling rate. I think it's like 2 GT/s, while GDDR6X is at 20 GT/s (both per pin). PCBs aren't too slow for HBM2, a silicon interposer is just the only practical way to route a memory interface using something like 7000 pins directly between wafers. If HBM2 chips were using regular packaging and pitches suitable for PCB use, each stack would have to be a huge package with thousands of balls.