If I understand correctly, this is purely a policy level decision to accept first-seen or a secondary deterministic test, but the most-work chain is still always better than a "more fit" but less work chain.
In any case, I'm skeptical of the properties of this change. First-seen has a nice property that once you receive a block, you have a substantially reduced incentive to try to orphan it because the rest of the network is going to work on building on that block. With fitness, I have a 50% shot if I mine a block of mine being accepted, so floating point would have the effect of destabilizing consensus convergence at the tip.
I could see using a fitness rule like this be useful if you see both blocks within some very small window, e.g., 10 seconds, as it could decrease partition risk if it's likely the orphan was mined within close range of the other.