He's not making a comment on voting or not voting at all, in fact this is written after Boutwell was elected.

He's addressing criticisms that directing protests at Boutwell before he has a chance to govern is misplaced and ill-timed, and he's pointing out that while Boutwell may be gentler, he's still a segregationist and is still in need of pressure. It doesn't matter if one is gentler than the other, the goal remains the same, and no freedom is ever given by the oppressor without being demanded.

Biden is gentler, but he's still a Zionist, and so he is still in need of pressure.

In case you're unfamiliar with the rest of his letter, he's also saying that the purpose of all direct action is to place pressure on moderates so that they may come to the negotiation table, even -and especially- direct action that causes material (in MLK's case, non-violent) harm to those same moderates.

We know through painful experience that freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed. Frankly, I have yet to engage in a direct action campaign that was “well timed” in the view of those who have not suffered unduly from the disease of segregation

And I haven't even gotten to the Malcom X quotes.