The default access manager is not 'Default'. It is 'Lbt'.
static ns3::GlobalValue g_channelAccessManager ("ChannelAccessManager",
"Default, DutyCycle, Lbt",
ns3::EnumValue (Lbt),
ns3::MakeEnumChecker
(Default, "Default",
DutyCycle, "DutyCycle",
Lbt, "Lbt"));
Seemingly, 'Default' is a poor name choice for something that is not
actually the default.
But I think it explains that you are using Lbt if you didn't touch this
value.
- Tom