Do you have a link to hand? Does it depend on the GPU in the Mac mini? I use the TB port for a 2560x1440 U2711, and HDMI for a 1920x1200 U2412. But didn't think I can run two 2711's off the i7 mini.
I don't know about running 2 monitors at 2560x1440, but for getting 2560x1440 from HDMI,
https://forums.macrumors.com/thread...-support-2560x1440-through-hdmi-port.1407732/
You need to read through the entire thread as some posts will just repeat the official party line on the HDMI limits. But post #15 is where the OP states that it works. There appears to be caveats in terms of refresh rate.
Another one:
https://forums.macrumors.com/threads/mac-mini-2560-1440-resolution-over-hdmi.1765861/