They're still one relatively teeny investor in a company with a gargantuan market cap. Nobody really cares that much whether they hold or sell.

More likely, though I haven't read their letter in depth, is that their beliefs actually aren't that far out there - I mean, you probably aren't going to get pushback from anyone if you say "Google is bloated and needs to trim and focus". So they write a letter and make some noise, and then when Google makes some rather unsurprising changes they can say something like "look how influential and prescient we are", and that has value in the investment world.