I'm talking about free test blogs hosted right here on wordpress.COM - 3 of them.
timethief has two sites here, both with custom domain names and both fully verified with Google and the sitemaps for those two sites are fine with Google. They have no problems with them.
I just checked three client's sites here, one with custom domain name and two straight up free sites, and in their google webmaster setups they had not set up the sitemap, so I just did that for them and they was accepted without error.
Seriously, I don't know what is going on, but wordpress.com sitemaps are not the issue. I've also checked three client's self-hosted sites that are using three different sitemap generator plugins, and the headers on those generated sitemaps are identical in structure, and the information provided, to the ones that wordpress.COM produces.
There are probably 25+ million public blogs here on wordpress.COM (30 million total) all using that same sitemap. If this were a "problem" believe me there would be more than two people here in the forums going on about this.
Note the opening paragraph in the google link that the OP provided:
Google can accept Web Sitemaps in a number of formats, but we recommend creating a Sitemap based on the Sitemap protocol because the same file can be submitted to the other search engines, such as Bing and Yahoo!, that are members of sitemaps.org.
For the record, Bing and Yahoo both accept wordpress.COM sitemaps just as they are.
From that same google link.
In addition to the standard format above, Google also accepts the following as Sitemaps:
RSS, mRSS, and Atom 1.0
Yes, Google even accepts plain text files with no header information at all, just a list of URLs, one per line, that you would manually update.
Given my experience with Google doing web development, I can tell you that its fickle nature is a "feature" of Google and you can never tell from day to day what it is going to throw at you. I've had it reject its own meta tags before only to wait a few hours and submit the same number and it takes it.