Time Travel Logic Strikes Again

In James Blish's Midsummer Century (London, 1975), when John Martels falls into a new radio telescope, the apparatus accidentally generates a field that projects Martels' personality to 25,000 AD, when it is picked up by a suitable receiver. When Martels has helped the men of that era to defeat their evolutionary enemies, the Birds, he is told that:

(i) they can return his personality to his body the moment before it fell;

(ii) his knowledge acquired in the future will return with him;

(iii) he will not slip (because he will know not to try to climb down?);

(iv) but " '...your additional knowledge will last only a split second...'" (p. 104);

(v) "You will never come to our century, and all the gains you have made possible will be wiped out.'" (p. 104)

But he is in their century! I think that (i)-(iii) make sense if Martels is returned to the past of a divergent timeline (timeline 2). In timeline 2, there is no reason why he should not retain his knowledge so (iv) is wrong. (v) is true in timeline 2 but not in the original timeline (timeline 1). In timeline 1, he did come to the future and help humanity against the Birds. It is in that timeline that this conversation is taking place.

Like The Time Machine, Midsummer Century is a good short novel or long story about travel to the future but it would be conceptually better, in my opinion, if this conversation were to be revised somehow.