This is one aspect of the Thai Forest Tradition I’ve never understood, either. I sometimes wonder if Thai Forest monastics are even defining stream entry the same way the suttas are.
MN 44 quite plainly defines “sakkaya” as “the 5 grasping aggregates.”
UPDATE: since you brought up Ajahn Dtun, @Vic, his description of stream entry is mentioned here. His description of stream entry as " one who, to some extent, has let go of attachment to the body by clearly realizing that this body is not the mind and the mind is not the body" doesn’t seem like EBT stream entry at all.