Except that mindblank is an 8th lv spell, while polymorph can effectively grant mindblank at 7th lv (by transforming say, the party barb into a treant).
I don't think he is saying that virtually immunity to mind-affecting abilities is bad, but rather, the ease with which it can be obtained (FR has a LA+2 plant race, while woodling template is LA+3, for instance). Mindblank can only be accessed by spellcasters of at least 15th lv, which severely limits the number of people who can benefit from it.
Not to mention that this is a convenient solution to plants generally having poor will saves.
I just assume this has already been factored into their cr though (for example, the treant has a fairly high cr of 8, despite having only 7 HD, and I assume plant traits account for part of that).