Quoting: Trevorchef
Dermott cannot go to the taxi squad because he has to clear waivers and would most certainly be claimed.
Lehtonen does not have to clear waivers.
The Leafs are 566K under the cap with a 21 man roster including three goalies. They can't add another defensman's salary without going over the cap on opening night. (Unless they remove a forward, which seems unlikely at this point).
That is why Dermott is in at this point. Hope that clears it up for you.
Keefe made clear why Dermott was in - Lehtonen was not good enough in camp:
“We think there’s other areas of his game that need a little bit more time. We don’t want to rush him into a situation that he may be not adjusted to here quite yet. So, we’re gonna take our time with that.”
“We’re gonna take a little step back with Mikko and give him more time to watch and get a feel for the league,” Keefe said, “and then also more time to work on some things directly in practice and make some adjustments there. And then, we expect that he obviously will get in at some point and he’s more prepared if that’s the case.”
The write up at The Athletic was also completely correct (except to those who have rose-colored glasses with every free-wallet the team brings in):
"Pick through the scoresheet (was there a scoresheet?) and you might think Lehtonen had a great night."
"Lehtonen had trouble moving the puck cleanly to teammates and occasionally looked a step slow in his own zone."
"Sandin, for one, looked like he belonged in the NHL with his play in the scrimmage. He dished the puck crisply and cleanly in a way that Lehtonen did not. Can Lehtonen handle himself physically in his own zone against NHL-sized competitors?"