They do acknowledge that in the notes, referencing one other study (n=3) with one healthy aged author that did not exhibit these signs.

Relatedly, as this is a single-case study, without a control author, it remains possible that the decline in lexical diversity reflects natural ageing. But note that previous research [15] has found that healthy authors can maintain stable linguistic diversity into their late 80s, suggesting that the decline observed in Pratchett’s TTR is indicative of pathology rather than typical ageing.

IMO still all very inconclusive but an interesting avenue to explore.