Coinbase Sends American Clients IRS Tax Form 1099-KIf you are an American client of Coinbase and made plenty of transactions during 2017, you better check your email. The company sent out tax forms for 2017 to many clients who were not necessarily expecting it.
The post you quoted for 2018 has a news linked that tells about Form 1099-K being sent to clients for the year 2017 and not meant for taxes on gains in 2018 (news was posted in Feb 2018) and I did took some time to find other sources about this one and Google is only showing me the results of the news link above. If you are worried about missing to file your tax report for 2017 I think you need to check your email because that is where they will send you a copy of the Form 1099-K, if you haven't received any email they have a copy
here where you can download it.