i don't understand rabbi zeira's question. the problem with bringing korban todah on sukkot (chol hamoed) has nothing to do with melachah on moed it has to do with a specific learning (a hekesh between 3 regalim) so what is that kushyah? it seems to be based on the premise that the reason not to bring shlamei todah on sukkot has to do with melacha on the moed?!
shalom, israel
Dear Shalom,
Thank you for your question and sorry for the delay. I think this is the explanation.
If this Hekesh were true it would be teaching us that we cannot bring Nedarim and/or Nedavos on Chol ha'Moed. This would mean that such a Melachah is forbidden on Chol ha'Moed. If so, R' Shimon should also prohibit chopping wood on Chol ha'Moed, yet we don't see him stating this.
Kol tuv.
Y. Landy