I gotta be honest, this one perplexes me. There are 2 ways it can be OPI via Article 2 or Article 4 below. I don't think you can say the SF guy was blocking so all I can think the refs are claiming is that the contact was just incidental but the receiver wasn't playing the ball at all and you can claim either of the 2 parts of Article 2 I list below.
RULE 8 FORWARD PASS, BACKWARD PASS, FUMBLE
SECTION 5 PASS INTERFERENCE
ARTICLE 1. DEFINITION. It is pass interference by either team when any act by a player more than one yard beyond the line of
scrimmage significantly hinders an eligible player’s opportunity to catch the ball. Pass interference can only occur when a forward
pass is thrown from behind the line of scrimmage, regardless of whether the pass is legal or illegal, or whether it crosses the line.
When the ball is in the air, eligible offensive and defensive receivers have the same right to the path of the ball and are subject to
the same restrictions.
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Offensive pass interference rules apply from the time the ball is snapped until the ball is touched. See Article 2 for prohibited acts
while the ball is in the air and Article 4 for prohibited acts prior to the pass.
ARTICLE 2. PROHIBITED ACTS BY BOTH TEAMS WHILE THE BALL IS IN THE AIR. Acts that are pass interference include,
but are not limited to:
(a) Contact by a player who is not playing the ball that restricts the opponent’s opportunity to make the catch;
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(e) Cutting off the path of an opponent by making contact with him, without playing the ball;
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ARTICLE 4. OTHER PROHIBITED ACTS BY THE OFFENSE. Acts that are pass interference include:
(a) Blocking more than one yard beyond the line of scrimmage by an offensive player prior to a pass being thrown. See 8-3-1-
Note for exception for an ineligible offensive player; or
(b) Blocking a defender beyond the line while the pass is in the air if the block occurs in the vicinity of the player to whom the
pass is thrown. See 8-3-1-Note for exception for ineligible players.