Chasing the market is usually a bad idea. Unless you're an extremely fast writer, the trends may have changed again before you get there. In this case, the likely motivation is that the market is (currently!) saturated with series, and readers (and publishers) already have their hands full keeping up with the series they've already committed to.
There's no particular magic to not being a "debut writer" --in fact, having been published with an unsuccessful book can even hurt you. What they're really looking for is writers with proven appeal, or pre-existing fan bases. If you already have a strong manuscript ready, shopping it to a niche publisher instead of leading off with your series might not be a terrible idea (due to the saturation of the market). And if you haven't started work on either book, I might recommend going with the standalone. But I wouldn't advise writing a book solely for that purpose.
Your best bet is always to write the book you are most passionate about --to lead with the work you think is strongest. A strong book can buck the trends. A mediocre book is unlikely to succeed, even if it's on-trend.