The "can do it" refers to the question "could we have the SOPA censorship label up on HN's logo today?" (Similar to what was done at http://www.avc.com for example.)

The comment I am commenting on says:

"if google, yahoo, ebay, reddit, mozilla, zynga, AOL, twitter, facebook, linkedIn, etc can do it then i can't see why HN (being all about dev and startup community) can't do it."

So "can do it" doesn't refer to signing a letter (which can't be done now anyway) it refers to putting the censorship label over the logo. The majority of the companies mentioned did not do that which was the point that I was making.