Lifnei Iveir - Bad Advice or Causing an Aveirah?

The gemara in several places says that if one enables another to do an aveirah it is considered lifnei iveir lo siten michshol. For example, in Pesachim 22a it says that if one hands a cup of wine to a nazir that is considered lifnei iveir. The question is what happens if I assist someone in doing an aveirah midrabbanan. Is it possible that even though I aided in a drabbanan that I am guilty of a d'orayssa? This is actually a dispute amongst rishonim [see Tosafos, Avodah Zarah 22a, d"h tepuk ; Minchas Chinuch 231:3 (in the hashmatos); and Sdei Chemed 9 : 36 (p .6)]. Some argue that, indeed, the aider cannot be worse off than the one he aided.

However, others argue that even if the aveirah is d'rabbanan - still the lifnei iveir is d'orayssa. How can this be?

An interesting approach I saw* is that it depends what the issur of lifnei iveir is. If you say the that main nekudah of lifnei iveir is not to cause someone to sin - so of course your sin cannot be worse than the sin you caused. However, if the issur of lifnei iveir is simply to not to give someone bad advice. An issur d'rabbanan is also bad advice and your aveirah can very well be a d'orayssa.

*I saw this explanation in the Journal of Halacha and Contemporary Society Volume 19 - see the article on "Enabling a Jew to Sin" by Rabbis Michael Broyde and David Hertzberg in footnote 14.