Deciding whether business came before pleasure, as the old line has it...
In the version that I have access to (via the Internet Archive, being a cheapskate who's not bought the actual book), he says the following on this:
Of some interest in this connexion is a confidential report which we received just before the outbreak of war on the subject of Poland's allegedly offensive intentions. It emanated from a source - hitherto regarded as completely reliable - in the immediate circle of either the Polish President or Marshal RydzSmigly, and contended that the Polish deployment would be offensive in character and include the concentration of strong forces in the province of Poznan. Most remarkable of all was the allegation that this plan of campaign had actually been proposed, if not demanded, by the British! In the circumstances we found the whole thing rather improbable.
I'd suggest that Manstein didn't take the claim that seriously, but as you note, he wasn't particularly complimentary about the way in which the UK and France appeared to have left Poland in the lurch, as he argued that without the Allies intervening, all Poland could do was play for time. He was also, of course, not particularly impressed with the plans the Poles followed, noting that Weygand's suggested deployment was 'the only proper recommendation to make', but that as this involved giving up Silesia to the invader, it was politically impossible.