Yeah, the Malthael’s comment was unnecessary, for sure. But whether or not the boss was the right call depends on too many things we do not know, so without the replay, the best I can do is reconstruct what I can from the few details in the OP.
What we know:
It was a map with a boss, likely a single boss based on the description. My guess is Sky Temple based on the draft picks we know, but it could be Tomb of the Spider Queen.
The OP’s team had a Malthael, a Valla (no idea which build), a Murky, a tank of some sort, and I would guess an unnamed healer. The healer may have been dead, given that no mention was made of them at all.
It may have been early game, or the enemy team probably had structures up, given that Malth and Valla were clearing a wave close by. I am discounting the time stamps in the OP as people are notoriously bad at estimating lengths of time.
We don’t know the enemy team comp at all.
With this in mind, a boss call was likely not the best decision, as their team has bad boss racing with a Malthael and Murky, so even if they came over, it is unlikely that it would be down fast enough to get any value in lane. It might have been doable if the Valla went Q build, but given that it doesn’t sound like a good Q build map, it is more likely she went Multi-shot if she was good, AA build if this were Gold or lower. If the OP were playing a Sledgehammer Muradin, it might have been doable.
Although they don’t have great push with their comp, even just getting some towers down makes a difference. Especially on the two maps this could have been, the boss area is known as the “throw pit” for a reason!
Bosses can definitely be a start to a snowball, or a win condition, but from the description in the OP, there just isn’t enough information. When I am tanking, if my team wants to do a boss, I will follow along as it is better to do the wrong thing as a team than the right thing on my own. However, in that situation, I probably would have pinged the structure and tanked some tower shots for them.
If the OP has the replay, it would be very interesting to see what actually happened, as for all we know, the boss may have been the right call.