Kesamim d'rabonnon

Shmuel said in nidda 57b that if a woman sat on the ground then she is tehora and he quotes a pasuk that proves that a woman is not temeia if she doesn't have a hargasha. The gemara quotes many proofs to show that kesamim are tamei so Shmuel can't dispute that. R' Yirmiya M'difti says that Shmuel only meant that she's n0t temeia d'oraysa but kesamim are tamei m'drabonon. Rav Ashi says (on 58a) that Shmuel holds in this case that she's not temeia because the kesem was found on the ground which isn't mekabel tuma.

Tosafos understands that Rav Ashi isn't arguing with R' Yirmiya M'Difti that according to Shmuel kesamim are d'rabonon. He's just extending it - the only time she's temeia d'oraysa is if she had a hargasha but kesamim aren't even tamei m'drabonon unless it's on a davar shemikabel tuma. Tosafos explains the reason is since the place where you found it can't become tamei from the dam so they weren't gozeir tuma on the woman either.

Rashi explains Rav Ashi that he's saying that Shmuel wasn't saying anything about hargasha at all. It's still clear from Rashi though that Rav Ashi agrees that according to Shmuel kesmaim are only d'rabonon. I'm not sure if there is a machlokes at all between Rashi and Tosafos but the Acharonim (the Aruch Laner deals with this in depth) try to figure out how Rashi can say that it has nothing to with hargasha when Shmuel followed his statement by quoting a pasuk to prove that the woman is only temeia if she had a hargasha.