Yes, this is what my experiments showed, and I’m suggesting it to be changed if possible, or the messaging in the GUI improved. 
My point is that this is surprising to me. The text in the GUI did not imply this at all. My understanding was that when, say, a finished workout comes in with both power and HR metrics, those settings allow me to tell intervals.icu which one it should use in its calculations. So far so good. I understand that, I understand the necessity.
But what I found out is that for a planned structured workout, intervals.icu enforces to use either power or HR for the whole thing; it will not let me alternate laps, each with either power or HR, and send them as such to the device. It artificially removes either the HR or the power intensities now.
My question is: is this a limit imposed by the FIT file format that is generated for the device? Is it simply technically not possible to have one lap with power and another with HR? If so, then cool - in this case this should be highlighted in the GUI in some way. English is not my primary language, maybe it’s an understanding issue on my side - in that case it would be great if the message could be made clearer. An explicit big fat red “one session cannot contain both HR and power laps” warning, especially within the builder itself, would work wonders.
OTOH, if it is technically possible to emit a FIT file where one lap is with HR and another lap is with power, then it would be great if intervals.icu would do so. So far, my understanding of the FIT format was that the intensity type is per step, not a single global setting for the whole file.