Are these tomatoes San Marzano knockoffs?
It's not a knock-off, per se—but this particular brand has never been imported from Italy.

"San Marzano" is a variety of plum tomato, as well as a protected designation of origin for those tomatoes grown in a specific region and in a traditional way. There is no single "San Marzano" brand or trademark owned by a particular company, and the name is not legally protected in the USA; instead, products originating from the traditional region are indicated by trademarked "DOP" certifications. 

Your can is produced by Simpson Imports, the same company that made the San Marzano canned tomatoes with the recognizable design that you're familiar with, which has always been grown in the USA—so in that sense, your can is not a knock-off. However, historically, the brand has probably benefitted from confused customers who thought they were buying tomatoes from the traditional region. The "SMT" design appears to be a newer rebranding; without actually knowing the motivation, it's possible that it may perhaps be to try to establish a trademarkable name, or to incorporate non-San Marzano varieties of tomatoes in their products, or to respect the designation protection and reduce consumer confusion, or perhaps some other reason.

Old label:




New label:



(This label is for puree tomatoes instead of whole, so there are additional differences beyond just the rebranding.)


Brand website: http://www.simpsonbrands.com/
San Marzano tomato drawing (but not "San Marzano" name) trademarked by Simpson Imports


Information about the San Marzano protected designation:


http://slice.seriouseats.com/archives/2010/12/what-is-a-dop-italian-san-marzano-canned-tomato.html
https://livethelive.com/2015/08/14/san-marzano-tomatoes-real-or-fake-3/
http://www.pastene.com/san_marzano.html
https://www.gustiamo.com/san-marzano-2/