Homebrew questions in general, and that question in particular, are probably fine.
There are a lot of reasons why people might create homebrew content. Some people simply enjoy the intellectual process of creating homebrew (and often then release it, either via free or paid means, to the public). Some people are DMs and make homebrew to help a player play exactly the kind of character they want. And some people, as in the question you referenced, are themselves players trying to make exactly the class they want.
Fundamentally, it's only that final group that is likely to run into problems. People who are DMing or just want to create homebrew rather than use it themselves don't need anybody's permission to do what they want with their homebrew. As the answer you mentioned pointed out, just because we here at the Stack think a piece of homebrew is properly balanced and suitable for use doesn't mean that the player's DM will agree, potentially invalidating their reason & effort for making the homebrew.
Even in these situations, however, the site's assistance is not automatically useless to the asker. Giving the player the ability to tell them DM "I worked with a group of people online to balance the homebrew, you can find our discussions [here]" may well influence the DM's decision about whether to allow the homebrew.
Further, there is a somewhat more philosophical aspect to this: The Stack's purpose is not to investigate & judge an asker's reasons for asking, it's to answer what was asked. Sometimes we need to tease out why someone asked so that we can give them a better answer (especially in the case of XY problems), but "is this homebrew balanced" doesn't really fall into that category. When we notice that an asker is creating homebrew they want to use as a player, there's no harm (and probably even some benefit) in helping them understand that no matter how much we help them it's ultimately the DM's decision, but we can still help them evaluate and improve their homebrew regardless of whether their DM approves or whether they ever even actually present it to their DM for consideration.
Finally, it's worth noting that the Stack's goal is not just to help the asker, but also future readers. Just because an asker doesn't manage to get the homebrew we helped them refine approved by their DM doesn't mean that the homebrew itself, or the issues we identified and the solutions we found, won't be useful to someone else in the future. Our duty is just as much to the long tail as it is to the asker.
