Sorry for the delay to respond, I've been busy last couple weeks.

I think you've convinced me regarding this. To discuss my own perspective on this, in the past it took me quite a while before I 'got' why one-boxing is the right decision in Transparent Newcomb -- it was only once I started thinking of decisions as instances of a decision theory/decision procedure that I realized how a "losing" decision may actually be part of what's a "winning" decision theory overall -- and that therefore one-boxing is the correct strategy in Transparent Newcomb.

I guess that in Ultimate Newcomb, one-boxing remains a winning decision theory, though again the winning decision theory is represented in a seemingly 'losing' decision. That I failed to get the correct answer here means that though I had understood, I had not really grokked the logic behind this -- I behaved too much as if EDT was correct instead.

Thanks for guiding me through this. Much appreciated!