Oh no!
To my knowledge, all elements in yUML are part of the OMG specification.
It’s possible to create non-standards compliant UML with many modelling tools, yUML included. It doesn’t enforce strict standards. For example, you could put inheritance at both ends of an association! So it’s up to you the author to be as strict or relaxed as you like.
In the industry many teams use UML to share ideas in a throwaway fashion, or for illustration. It’s not always about perfection, but instead communication.
Do you think he’s spotted some diagrams that break the standard? If you want me to scan your thesis in confidence I can see if I can spot any errors. Or even better would be great if he can point out which diagrams he feel break the standard and either you can fix it or I can fix yUML.
Hope this helps.
Tobin