A solution that I found through experimentation.
Apparently, in the Table() constructor for schema extraction type coercion does not happen for custom TypeDecorators. To solve this, you do the following:
table = Table(table_name, meta, Column(column_name, custom_type_Decorator), autoload=True, autoload_with=sengine)
This is unfortunate since I thought reflection would pick up this dependency in one of the columns.
Not sure why this was not answered after a few days. I guess new members are at a disadvantage when asking questions in accordance with the gamification rules system stackoverflow is using.