Just define a column constraint (syntax might be wrong):

CREATE TABLE t (col TEXT CHECK length(col) < 50);

What is nice is that you just need to add/drop a constraint if you want to change the max value check rather than changing the data type which can result in a table rewrite depending on the scenario. You can also add a min length check as well using this method or regex check, etc.

I think the author should have made this point rather than just glossing over it with "constraints, triggers are more flexible".