I'm fairly sure that the link is what might be described as thematic. That is, The Amazing Maurice and His Educated Rodents features a boy whose main companions are a group of small non-human people, many of whom regard thievery and deceit with equanimity, and of whom the adult world is largely unaware, and the Tiffany novels feature a young girl whose main companions are a group of small non-human people, many of whom regard thievery and deceit with equanimity, and of whom the adult world is largely unaware.
Keith is the 'stupid looking kid' who people are prone to underestimate, and Tiffany is a girl; hence prone to being underestimated.
Keith's story takes place entirely away from Ankh-Morpork and the earlier Tiffany books do likewise.
In addition to this, I think it may be a case of the author of the reading order striving to include all the discworld novels in their schema and there being no stronger or more explicit links than a general thematic one as I propose.
As you have mentioned Malicia, I think there is also a thematic link there, in that the L-Space wiki describes her as 'a girl who asks questions' which would also describe one of Tiffany's defining characteristics.
As there is a gmail address for the originator of that reading order included on the image you linked, I will drop them a line with a link to this answer and invite them to let me know if they disagree or have anything further they would wish to add.
The response from the reading order author reads as follows:
Hi, there is no link other than these are young reader novels.
So, so much for my literary analysis!