According the the rules, if the offensive linemen make initial contact within one yard and maintain that contact they are legally downfield. Watching the video clip, the Eagles linemen establish their blocks at the LOS and push the Cowboys backwards. None of them continue to move forward after breaking contact with the Cowboys defenders. That makes them all legally down field.
From the rulebook:
SECTION 3 - INELIGIBLE PLAYER DOWNFIELD
ARTICLE 1. LEGAL AND ILLEGAL ACTS
On a scrimmage play during which a legal forward pass is thrown, an ineligible offensive player, including a T-formation quarterback, is not permitted to move more than one yard beyond the line of scrimmage before the pass has been thrown.
Item 1. Legally Downfield. An ineligible player is not illegally downfield if, after initiating contact with an opponent within one yard of the line of scrimmage during his initial charge:
(a) he moves more than one yard beyond the line while legally blocking or being blocked by an opponent
(b) after breaking legal contact with an opponent more than one yard beyond the line of scrimmage, he remains stationary until a forward pass is thrown
(c) after losing legal contact with an opponent more than one yard beyond the line of scrimmage, he is forced behind the line of scrimmage by an opponent, at which time he is again subject to normal blocking restrictions for an ineligible offensive player.
Note: If an ineligible offensive player moves beyond the line while legally blocking or being blocked by an opponent, an eligible offensive player may catch a pass between them and the line of scrimmage.
Item 2. Illegally Downfield. An ineligible offensive player is illegally downfield if:
(a) he moves more than one yard beyond the line of scrimmage without contacting an opponent
(b) after losing contact with an opponent within one yard of the line of scrimmage, he advances more than one yard beyond the line of scrimmage
(c) after losing contact with an opponent more than one yard beyond the line of scrimmage, he continues to move toward his opponent’s goal line.
(d) Penalty: For ineligible offensive player downfield: Loss of five yards from the previous spot.