A school photograph (shown on the cover of the book) shows Hitler and Wittgenstein standing next but one to each other.
The school had 329 pupils - not a huge school , and of a size when pupils would likely know each other.
They were the same age, although not in the same class, as Wittgenstein was a year ahead and Hitler a year behind the average (p.10).
A pupil records that Hitler called someone a "filthy Jew" who at the time did not realise he had Jewish ancestry, this description fits Wittgenstein.
In Mein Kampf Hitler recalls a Jewish boy "who was treated by us with caution, but only because various experiences had led us to distrust his discretion and we did not particularly trust him". Wittgenstein is well known for his "confessions" and his obsession to tell the truth, again making the link between Wittgenstein and Hitler at school.
Wittgenstein’s family was incredibly rich and powerful, hence Wittgenstein because of his family alone would have been well-known to the other pupils. However in addition Wittgenstein was a "small, unathletic, stuttering, homosexual, adolescent" (p.18) which means it is inconceivable Hitler could not have known Wittgenstein.
Both Hitler and Wittgenstein loved Wagner (p.12-14) and were both able to whistle large sections of his music. This common interest not shared by the other pupils again makes it likely they would have known each other.
Wittgenstein referred to other pupils as "muck" and spoke down to them using the term "sie" (p.18). It is on record that Hitler also referred to other pupils as "sie" (p.21) and this term was also used in later life by Hitler (p.22). Their both speaking in the same manner again points to a connection.
Both pupils were major figures of the twentieth century, it is likely that as children they would have stood out and hence be known to each other (p.18).
Wittgenstein had two homosexual brothers who had killed themselves. This would have made him widely talked about at the school (p.30), again showing Hitler must have known of him.
Wittgenstein’s family financially supported anti-Wagner artists and musicians. As a lover Wagner, Hitler would have known this and resented Wittgenstein for it.
The descriptions Hitler in later life gives of Jews actually fits Wittgenstein: the outward appearance of being european (p.23); bearing titles of nobility (p.23); being ‘court Jews’ (p.25); writing for the world press (p.27); spending the night in the Hotel Excelsior (p.30). Even Hitler’s laws defining who was a Jew (three of the four grandparents had to be Jews) seemed especially to be written for Wittgenstein (three of his four grandparents were Jews). This shows Hitler had Wittgenstein in mind when he pursued his war against the Jews.
Another reference Hitler makes to the Jews is "of German he possesses nothing but the art of stammering its language" (p.23) - Wittgenstein was a stammering Jew, hence again Hitler makes specific references to Wittgenstein into general attacks on the Jews.
Statically there few Jews at the school, hence Hitler’s references to Jews at the school make it more probably he was referring to Wittgenstein or had him in mind.
Hitler actually refers to Wittgenstein in a speech following invasion of Austria, when he says "would that on this evening, some of our international seekers after truth whom we know so well could not only see the facts but later admit them to be facts". "Seekers after truth" is a reference to a philosopher whom Hitler knows well - Wittgenstein.
Also some nice discussion on what Cornish might have been trying to do by making such a claim.