According to what you have posted from Al-Tabari, it is acknowledged that “others said that it is night of the 15th of Sha’baan” … there is nothing in this commentary saying that those “others” are shi’a and it further implies that there is a ‘difference of opinion’.
The versions I have heard are that there are at least three “blessed nights”, one of them is on the odd nights in the last 10 days in Ramadan, one of them is in Sha’baan and another is the al-isra wal mi’raj.
To be confined to one opinion and impress on others that ‘bid’a’ is the only result in a belief other than your own, despite it being present in sources is itself ‘intolerance’ and by proxy an act of ‘slander’. It is better to accept the difference of opinion.