פירוש על ידים 3:3

רצועות של תפילין מטמאות את הידים – that when they (i.e., the Rabbis) made the decree on [the defiling of] the hands that comes on account of a scroll that invalidates the heaven offering, they decreed also on the hands that came in contact with the straps of the Tefillin/phylacteries.

The straps of the tefillin [when connected] with the tefillin [boxes] defile the hands. Tefillin boxes have scrolls of Scripture in them. Therefore, they clearly would defile the hands. According to the first opinion, the straps do as well, as long as they are tied to the tefillin boxes.

ר' שמעון אומר כו' – But the Halakha is not according to Rabbi Shimon.

Rabbi Shimon says: the straps of the tefillin do not defile the hands. Rabbi Shimon says that the straps do not defile the hands, since they are only connected to the boxes they don't have the scrolls in them.