If microsoft does not write the operating system for the Macca produced ECU, who does? Linux? Steve jobs? Mr. Coughlan?
Maybe... FreeBSD developers?

At least some early network exes in Windows were nothing but
adaptations of BSD code. And Microsoft surely didn't write those. I'm just joking here, by the way.
Seriously speaking, I think it's perfectly possible that McLaren are associate partners of Microsoft and, with some external support, the ECU software was mostly written by McLaren engineers. Due to licensing or whatever, or maybe because they used two guys from microsoft that wrote assembly code into the ECU, the source legally belongs to Microsoft and that's why it's called that way.
PS: I was going to write how the analysis you made on the syntax of "microsoft ECU software" is totally different in spanish, but I suppose this is not a language forum. The turning cog told me that.
OT: Am I the only linux user here?
And, yes, F1 coverage is horrible. Drivers are mantained far away from the fans, engineers are kept under rocks, least they say something interesting and TV broadcasts are really horrible. Come on, even in Golf they have protrace, to see how some golfers attack the pin using fade/draw. Why can't we have something similar for F1? Why can't we have some more serious analysis on performance?
I am not amazed by F1 cars in Monaco. I want to see them driving in the A8 highway: Variable radius corners, negative banking, and extreme narrowings that Tilke has never dreamed off. Oh, yes, and "beautiful" weather tops it all.
"Prediction is very difficult, especially about the future." Niels Bohr