Thank you for the explanation!I am not certain which sex link they are referring to exactly but it's probably the red ones.
Here is an article about it:
https://www.backyardchickens.com/articles/an-inside-look-at-the-humble-sexlink-chicken.73484/
I have never owned red sex link so I can't say if this really applies to all red sex links or just one hatchery has a line that has the problem. The idea is they have more problems because they have over-active ova-tracts. If you want a pet chicken to live 5-7 years don't get a sex link. You want an egg machine who will burn out and need to be replaced in a few years go with a sex link. Some birds where created with a purpose in mind: Cornish cross- meat bird and red/black Sex link - egg layer.
Not to say you can't get one of these to out live there target age, it's just that they where not designed to.