The section of the poem that you refer to does form part of the epiphany that the speaker of the poem experiences once she truly looks at the fish and sees the evidence of the previous struggles for victory that the fish had endured and triumphed over. Once she sees this aspect of the fish, her immediate response is just to "stare and stare" as "Victory filled up / the little rented boat." The quote you have highlighted indicates the way that even nature in the form of oil and bilge water in the boat and sunlight combining together parallels this moment of inner insight and epiphany with the rainbow that they form.
There definitely does seem to be a sense in which this epiphany is related to the speaker seeing something of the condition of humanity in the situation of the fish. Let us remember that the fish is being lifted "half out of water," occuping a kind of liminal location between his natural environment and the environment that will kill him. This situation is paralleled by the way that the speaker is on water, not her natural environment, and in a boat that is "rented." Perhaps she sees another parallel in the way that the fish has had to struggle so hard for life, just as we as humans face grim struggles in our own lives for survival. It is this that causes the speaker's identification with the fish, which is in turn mirrored by nature in the form of the rainbow that is created. Her act of letting the fish go seems to challenge the idea of victory entailing a winner and a loser by letting both the speaker and the fish share the victory at the end of the poem.