"It is exception from liability for non- performance rather than an exception from obligation to perform".
Similarly, Lord Winn says (ibid., at p. 147 v. A):
"As I construe the clause it affords only immunity against any claim for damages".
Lord Denning did more. According to his view, even if we assume that stipulation means the absence of a breach on the part of the company, even then the cause of the breach remains intact. In his words (at p. 137 vs. D-E):
"… and I am prepared to assume that ESSO would not be guilty of a breach of contract. But I do not think that would exempt the trade union officials from liability if they unlawfully hindered or prevented ESSO from making deliveries".
In my opinion, it is appropriate to adopt Lord Dunning's approach.
My conclusion in our case is that Knopf's release from liability for Ben Shachar's failure to leave on time is limited in scope to a claim for compensation for the damage caused due to the failure to deliver the leased property on time, but the breach in itself stands. However, even if you conclude that even the infringement does not exist, even then this does not change anything about Ben Shachar, whose duty is not to interfere with Knopf's delivery of the leased property on the agreed date.
Even if we had interpreted P/8 as a condition suspending the date of the entry into force of P/2 (and for this there is some support in the evidence material), my conclusion would not have changed, since even then this does not release a stranger from the duty of non-intervention: Salmond, "The Law of Torts", p. 396 (where it is explained that the duty of the stranger remains "even though the contract is regarded by the parties to it suspended rather broken...").
The situation might have changed if it had been proven that the initiator of the new lease with Ben Shahar was Knopf: see Prosser on Torts 4th ed. (1971) p. 934:
("acceptance of an offered bargain is not in itself inducement of the breach of a prior inconsistent contract…").