That's only the case if you add a DEFAULT value for the new column. Up until the soon-to-be-released v11 that required updating existing rows. Without a default it's essentially just inserting a single row in a row into an internal catalog table (pg_attribute). With a default in v11, we basically just store the default for a new column out of line, and reference it when the row is read.
In either case a lock has to be acquired on the table, which means if there's a longrunning transaction, you can end up blocking for a while...
In either case a lock has to be acquired on the table, which means if there's a longrunning transaction, you can end up blocking for a while...