Parents discovered sled tracks from home onto broken pond ice through which he fell.
He left the house at 16:00, which is why they give the range of 147 to 177 minutes.
What we are certain of is the 16:44 call to the EMS, so you're right, 134 minutes is the lower bound. However, it's not unreasonable to assume that the parents did not immediately call on noticing the child missing, so the gap between 'noticing missing' and 'calling EMS' is real and non-zero.
The gap between 'leaving the house' and 'noticing missing' is something I'm less clear on: how did the parents 'notice' the child was missing when he was knowingly allowed to leave the house earlier? There's still a non-zero gap between 'last sighting' and 'realisation that he's missing' (i.e. he definitely wasn't missing at ~16:00, when the mother saw him leaving the house... what changed so that the parents believed he was missing at ~16:30? Was he supposed to check in with Dad and missed the check-in, etc?)
Either way, a difference of ±10% doesn't really make that much impact in interpreting the results.
two hours underwater! that's insanity.