As they both result in the same thing (you need to read what he says understanding he may be biased), you'll see them both used. I usually use Disclaimer, but there's no real reason for that.
A disclaimer is a piece of text a person or company writes to avoid responsibility. In the GPL, there's text saying that the program is distributed without any warranty - that's a disclaimer. If someone sues you because the program broke, you can point to the disclaimer to show you never claimed that it would work.
Disclosure is just revealing information.
That parenthesized remark was disclosure (because the author revealed his affiliation with Microsoft) but it was also a disclaimer - because of that text, no one can claim that the author was astroturfing.
A minor nitpick: I thought that you call this "disclosure" in English, not "disclaimer"? (I'm not a native speaker, which is why I may be easily wrong, but something is nudging me to ask when I see things that seem wrong to me.)