I was given a DMCA takedown notice on my original content. Is their claim legit?

Counterclaim? Under the DMCA? What are you talking about? Just send them a letter or e-mail and ask the company what item in particular they feel infringes on their copyright. By the way, your not being targeted. Many people create similar products from scratch. However, if someone did it first, they win. Its not that you sought to copy their material, or even knew it existed, but your product may still be too similar and may infringe.