That's the letterhead of the sender. Pichai is the recipient, under that is his affiliation: Alphabet.

Edit: it's funny, I learned this in high school in a "keyboarding" class that was a complete waste of my time. At the time, I was annoyed: if I'm reading a letter, it's probably written to me; if I send a letter, the recipient will know that my name is not theirs. Where's the ambiguity? All these years later, it finally comes useful. I got an internet point.

Sure, but the argument is about if the letter was sent to Google's owner which it is not.

It is sent to Sundar Pichai, Alphabet's Chief Executive Officer which you have also pointed out but the grandparent had not figured out. Sundar Pichai does not own Alphabet in addition to having the role of CEO.