Long term battery degradation happens when its membranes degrade. There are hundreds of variables involved. Temperature IS a variable. But I'm reluctant to outright confirm what you're saying because it may be misleading. Avoiding plugging in when hot could help in some situations, but it won't help in all, so it's not a categorical rule.

Heat is a major cause of battery degradation, partially why I don't agree with constantly charging/discharging since it causes more heat.

Keep your laptop cool, perhaps even consider battery saving options while plugged in if you find it is generating too much heat.

Edit: A running laptop will cause heat, so shut it off when not using it. A plugged in laptop shouldn't create much more heat than a laptop running on battery power, if anything, the battery should be cooler when plugged in and fully charged (assumption made).