I don't think that is true...
Below is information that states they are legal to transfer. There is no felony in doing so.
Links included under the text.
"If a weapon is a copy or duplicate of one of the models enumerated in the law, it is an Assault Weapon. The Enforcement notice will not be applied to possession, ownership or transfer by an individual gun owner of weapons obtained on or before July 20, 2016."
www.mass.gov
The Infamous 7/20/2016 Enforcement Notice
On July 20, 2016 the office of the MA attorney general released an
enforcement notice declaring that the ban-compliant rifles sold in MA since 1994 were now "copycat" weapons, and in violation of the AWB. Since that date, dealers have been prohibited from transferring these rifles to LTC holders, but private sales are still allowed.
The table below shows which assault weapons can be transferred, and how.
This post explains the Asaault Weapons Ban in Massachusetts.
massgunownership.com