Agreed. Tgrep is a nice take home challenge, but a client-side Reddit clone is far too much to ask for. The only applicants they're going to get for the front-end position are Reddit mega-fans with too much spare time - though I suppose that may be what they're looking for.
Regardless, most real "rock star programmers" (as much as I hate the term) aren't going to be blowing a day or two on spec work.
Funny, to me writing a quick and dirty frontend for a reddit clone seems like a fun mindless 3 hours. Building a replacement for grep and profiling it until it's fast sounds like the kind of pain I pay other people good money to avoid.
Regardless, most real "rock star programmers" (as much as I hate the term) aren't going to be blowing a day or two on spec work.