First, it depends on what we mean by "day". If "day" is "when I get home from my real job", then that really means "the hour or two I've got after dinner and before bed". If "day" means "all day, this
is my real job", then that really means eight-plus hours.
I can pretty consistently do 2k words/hour while writing fiction, and I can maintain that for as long as I need to maintain it. That's rough-draft mode, of course -- it usually gets edited down to 1500 words/hour when I go through and start snipping and trimming.
If I got the job, I'd have no problem maintaining the word counts they needed, because I'd be leaving my existing job.

The 30k words-in-a-month thing could be challenging, since it'd be stuff I'd be doing after getting home from my current job, but I can always beg my wife for a free day or two over the next few weekends and just burn through a fair chunk of it -- in addition to cranking out 2k/night on the nights when I'm home before eleven.
For the marketing copywriting I do, things are usually slower, primarily because I'm constantly looking things up and checking facts. (And also because my job isn't exclusively copywriting -- when you have to break for twenty minutes to go carry boxes around or help with the horses, that cuts into the writing zone...)