Tシャツがもらえることは Google Developers のサイトにも書いてありました。
Publishing your first app
Making your first app for the Google Assistant available to users is definitely something worth throwing a party for, and to kick off the celebrations we'll invite you to join our growing Google Assistant developer community:
・To welcome you to our developer community, we'll send you an exclusive Google Assistant t-shirt. Make sure to wear it when you meet our team at events around the world.
・To support your efforts, we'll also credit the account associated with your app with up to $200 of Google Cloud services a month (applicable to any Firebase plan)- for a full year!
Google Assistant Developer Community Program | Actions on Google | Google Developers