If I lied and told you this was grown in 10 dKH, would you believe it?
I was in a hurry yesterday when I responded with just one word: yes.
It warrants a slightly longer response: yes, wallichii will grow perfectly well in 10 KH. It is a really good old school low-tech plant. It's been in the hobby for decades. I know people who grow it is rock hard tap water with nothing but capped dirt as substrate. I've grown it this way. I've also grown it in inert sub with no CO2, no ferts, and low light. I've also grown it over soil capped with gravel and nothing else.
It'll grow well under PPS pro or Seachem Comp level ferts at KH 10.
When grown under these conditions, leaf structure is really good, but color may not be bright red.
Bright lights, CO2, and rich substrate increases color. Super soft water + EI + Aquasoil + bright lights improves color even more, but then it grows annoyingly fast.
Lots of ways to skin this cat.
However, if the above pic was 10 KH + inert sub + high CO2 + high light + EI...well, then, I am skeptical.