Hey Kevin and welcome to GUS.
I think in order to answer that issue of whether you're gonna tear something by keeping the scapula retracted you just need to consider how most strains happen. If you retract the scapula then the muscles are in a contracted state but not under a position of lengthening or stretch. They may of course be required to resist lengthening under load but your training should get your ready for that. MOST injuries happen when the muscles are generating force while lengthening or during simple stretch while trying to resist lengthening. It is VERY unlikely to 'tear' a muscle during essentially an isometric action.
If you allow passive scapula protraction during a heavy deadlift the stabilizers will have to resist that which is MUCH more likey to result in injury or predispose one to problems in those muscles.
And a "shoulders back and locked postion" is synonomous with scapula retracted.
Is it possible to maintain scapula retraction during the heaviest deads? Of course not. Will the upper back undergo some rounding during the heaviest deads? Of course. But the majority of time you should be attempting to keep scapular retraction, depression, and t-spine extension.
When you get really stable and experienced you should be able to tell the difference between some harmless thoracic rounding while the LUMBAR is neutral. Remember that the thoracic is built for more mobility than the lumbar.
BUT and this is a big BUT, in order to learn that you have to start with the t-spine extension. Practice that for the majority of your lifting activity. Build up a lot of muscle in the upper and mid-back and a lot of stability in the lower back/abdomen. All that extra mass/strength in the upper and mid back will help resist injury. Keep in mind that lots of rowing and other back/pc stuff is required.
All the while you would be engaging in thoracic mobility drills, etc..
I want to be clear on the point that the shoulders are MOST stable in a position of scapula retraction/depression.
Actually what you read kinda reminds me of how people have told me they want to do a lot of loaded lumbar flexion in order to be ready in case the lumbar flexes under load. The idea is not to use provocative measures (potentially harmful) measures to get ready for dangerous situation. The idea is to AVOID dangerous situations.