sounds like a great way to weed out companies you don't want to work for

"I see you choose x but I would say the appropriate choice here was clearly y due to Z"

"I see your hiring process is flawed, good-bye!"

As long as you understand idioms, programming paradigms in general, and good patterns in specific languages (e.g. list comprehension in python, prototypal inheritance in js, metatables in lua, etc.) you shouldn't be afraid of showing off your stuff in X language.

I know good Java developers that have become really good front-ends.

Well Java doesn't have a stigma attached to it like COBOL or BASIC does. What about those of us who have terrible QBASIC code they wrote when they were 14 on their github/bitbucket/etc profiles? I'm not going to hide that; it's public because it's useful. (I intend to dig up more terrible code I wrote and put it up)

Ignore them.

Sample size one in a forum...

I do not see how sample size has any relevance. There was no claim.