Fencer: I read Hostage Chess games commented by the author and yes, this seems to be the correct interepretation.
But there is one more point in this rule (probably it was stated later and you know it) : if no piece is in the opponent's prison, then the pawn cannot move onto the last rank. It means that in this case, a pawn on the seventh rank is not even checking a king on the last rank.
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