I love to see that Google is on their top donors page every year, it shows their commitment to supporting open source and the tools they have used to build their business on.
Without putting motives into Google's metaphorical mouth - and because I'm sure your logic is likely correct - even if they didn't use these tools and didn't care about open source, the good will they get for being seen doing this would probably be worth the cost anyway if they pulled it from a marketing budget.
And yet you don't find any of the other companies we associate with "tech giants" there, nor on Freebsd's donors page [1], whereas Google is there every year.
So no, I really don't think it's that easy to dismiss it (even though that wasn't exactly what you were doing).
http://www.openbsdfoundation.org/contributors.html