If the latter you refer to is
http://heatherrosejones.com/names/breton/daviesearlybreton.html, she seems unsure whether the -iun in the name Adiunus is that same as the Old Breton prefix Iun- in other names, not that -iun is derived from *ad-ioun (rather *Ad-ioun would be derived from *-ioun).
Gary German, on the another hand assumes that the prefix Iun- is the same root as the -un- in Idunet (the names cited are actually recorded with -iunet and -diunet), following sources of the Welsh Dictionary, which has eidduned and Welsh eidduno as prefixed derivatives of *-iun- "wish, desire". The relevant Breton onomastic forms are Latinized *Adiunus (recorded in the genitive Adiuni; Welsh adj. eiddun desirable, eager), -iunet, ~diunet (Welsh n. eidduned "a wish, a desire …" from add- + *iuned); eiddunaw, addunaw cited by
Heather Jones represent the form with verbal suffix -aw for -o (also -af).