Yes right now the 600 points added by PNP will make the 50 points excluded anyway even if you say you have an offer now because you can have more than 600 in that section, Hence you won't see a change in points now by setting yes or no now, that is clear and I'm not even talking about it.
As I mentioned earlier when Ontario sent the EOI, if the offer letter question was answered as yes 50 points would have been added. Then it was why OINP chose to send you the EOI over someone else who might have got that spot had you not had the 50 points, now setting it to no is a change in employment circumstances. Of course, as I mentioned earlier both OINP and IRCC may choose to ignore and still give you the PNP or might choose otherwise, it is entirely up to them to decide.
I'm not trying to play spoilsport here or trying to preach ethics, I wish you good luck for the rest of the journey.