DFA delivers two products to ISPs; dark fibre and more recently, managed leased line. Remember, DFA isn't an ISP, they do not deal with end users. Each circuit is a dedicated fibre pair from a DFA pop and it's not shared between multiple premises like an FTTH installation. In addition, a DFA build is slightly more complex than an FTTH build, as every DFA link has an SLA (99.5% uptime and 4hr Mean Time to Repair), so it's in their interest to build it not to break. This is why we use them for businesses - you can rely on the quality, uptime and maintenance. That being said, if you're ok paying for a business line, you will get a premium service to your home.
But no, you cannot run an FTTH service on a leased line; or more-so, you wouldn't benefit cost wise from a contended L3 service over a premium dedicated link, as the access port cost per line is the majority of what you pay anyway.
Unfortunately FTTH is an entirely different product to a leased line; it's not their model or feasible within their network design. Yes, it's a fibre, but no the network is not designed the same way.