OKIE DOK!! Being transharmful is just identifying as doing/having done harmful things, or identifying as a harmful person, regardless of what the identifier would actually do or their own beliefs.
For example, someone identifying as a transabuser due to intrusive thoughts, past life memories, experiencing abuse, worries of being an abusive person, engaging in CNC or nc-cnc (and other kinks), being in an "abusive" relationship (ex, a map in a relationship with an aam, not being abusive but being perceived as such,) just because you can, etc.
Just look at it the same as a "harmful paraphilia," the identifier isn't harmful until they cause non-consensual harmful to another being.
Hope this helps? You can try asking some other transharmful folks on tumblr, I'll go check and see if any other transharmful beings have said this better and post a lil link l8r!!