First I think I could make the polite debate between you and Brad even more polite. I didn't get the impression Brad claimed you are a Marxist. I think he suggested that Doug Henwood is a Marxist. Your quoted post explained an objection to left neo-liberals and said it had some merit. I think that perhaps you and Brad can agree that you see some merit in Marx's arguments (in any case he sure does).
However, I have some rude things to say about your writing as quoted by Brad. I don't like the word "theories". I don't think that anyone including left neo-liberals needs a theory of change in order to achieve change. Trees have no theory of photosynthesis, but they grow all the same. You can't really be surprised if an appeal to theory in the context of politics made Brad think of Marx. In any case, your interpretation of Henwood's argument appears to be that there is no convincing explanation of how the policies developed by Yglesias at all will actually improve the world, since they won't be implemented.
" This means that vaguely-leftish versions of neo-liberalism often have weak theories of politics, and in particular of the politics of collective action. I see Doug and others as arguing that successful political change requires large scale organized collective action"
OK now I create straw Doug Henwood who argues that Yglesias (and Brad, C and D Romer, Ezra Klein, Larry Summers, David Cutler, and Jon Gruber) are off in their ivory towers designing policies which will never be implemented because they haven't thought enough about class struggle. The problem with Straw Henwood is too obvious even for a comment on a blog. The neo-liberals I listed have achieved political change (even though I resisted the temptation to include the neoliberal wonks named Barack Obama and Bill Clinton on my list).
It seems to me that your current post is an excellent critique of your earlier post. You explain Summers's theory of change -- he says the key is to get ahead by getting along and influence powerful people. I hazard the guess that this wasn't his first approach (OK I hazard dozens of vivid memories). Technocrats have power (by the definition of the suffice 'crat). They exist. Politicians seek their advice. This is the way political change has occurred (lest I seem to celebratory I think the most influential interaction of an intellectual of sorts and a politician who achieved change occurred when Arthur Laffer sketched a curve on a napkin).
Yglesias clearly has a plan. He argues that good policies are good politics -- that voters re-elect incumbents if things are going well even if the voters opposed incumbents policies when they were implemented. So, he argues, listen to technocrats -- we will tell you how to win re-election -- oh and also serve the people which, of course, is the reason you got into politics in the first place. Look what happened to B Clinton when he listened to Robert Rubin and raised taxes on Robert Rubin et al.
Now I don't know if there is any theory of history such that this plan is anything other than a pipe dream. But I do know that it works. Klein at least, hob nobs with the President of the United States. That president has achieved a lot (even if much less than one might hope). I'd say actual change trumps theories of change.