Liberalism has little to do with the claim that human beings are naturally good, and Locke has a good deal less to do with that claim. Locke provides a blurry version of the social contract theory that Hobbes laid out earlier, and this theory is based squarely on the notion that people are selfish, frightened, and potentially violent.
The state of nature is a war of all against all, he said, and in it, the life of man is solitary, poor, nasty, brutish, and short. For purely selfish reasons, people concede some of their liberties to a ruler who can gather the resources to punish theft, force people to keep their word, and commit resources to necessities which each man would rather let someone else pay for.
This is Locke's argument too, although he is a muddled writer.
In any case, it is the idea of a social contract, not claims of natural goodness, which influenced political thought. Locke's conception of mankind is simply that people are created by their environment--the mind is a blank slate at birth--and this has more to do with his arguments about perception and thought than any political claims.
Except, of course, the implicit claim that everyone is equal at birth.
The claim of the social-contract is that the king is a creation of the people or, in our day, to say that a government derives its just powers from the consent of the governed.
The only political philosopher who argues for the goodness of man, so far as I know, is Rousseau. He has no connection to political liberalism that I can see.
From the other Enlightenment authors, of whom Jefferson et al were great students, there is a similar notion--the perfectibility of man. Maybe that's what the author of this entry is trying to get at.
The idea here is that people can improve themselves and their society without the guidance of a traditional authority. It arises in response to the dominance of a land-owning aristocracy. It is more or less the ideology of the rising middle class.
You can see that the social contract and the idea of perfectibility are related. Both are attacks on conservative values, challenging the assumption that some class of gooder, smarter, righter people DESERVE to run the show.
The conservative idea: there is a set of people who own all the productive land and collect part of the harvests that other people produce, but this group of landowners contribute something essential to society. They are not grubbing for a living, so they can remember the past and think ahead. They preserve tradition, conserve resources for future generations, moderate the impulsive to-and-fro of little men's present worries.
By the time liberalism arises, the Church had allied with the men of property. And the word liberalism was, as I recall, originally a term of religious accusation--if people tried to think for themselves, they would become libertines: promiscuous, unproductive, ruled by impulse.
By that time, though, the French landed aristocracy was represented by fellows like the Marquis de Sade.
You evidently know, or think you know, a lot about liberalism and its history. What puzzles and surprises me, then, is that you go to all this trouble of writing on a "talk" page, which really doesn't matter in the grand Wikipedia scheme of things, instead of editing the main article. Moreover, the time it would have taken you to edit the main article would have been a fraction of the time it would have taken you to write the above, I imagine. And as soon as you had edited the main article, I hasten to point out, the above comments would have become pointless. --LMS
Part of a general revision which I'll submit once I've worked out its implications in my own mind.
Great, I'm looking forward to it! --LMS