Your argument is fairly asinine. When you fork an open source project under the MIT license you have an obligation to include the original license in all copies or substantial copies of the code. The author of the fork may also sublicense, which allows them to add new terms to the license, but not remove the original license.
Forking and/or copying files from the Spegel code base into the Peerd code base is permitted, but since the Spegel code base had a single license file covering the entire repo, then the onus is on Microsofts engineers to update the code they copied and include the original license terms, for example, by including something like:
// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
// Some code Copyright (c) 2024 The Spegel Authors, under MIT license
If your argument is that they aren't required to do this because the original code didn't have a license header in the file, then it would follow that you are arguing that the MIT license doesn't apply to the code that was copied, in which case Microsoft is using unlicensed code stolen from an open source project.
While I haven't worked at MS specifically, I would assume that like every other tech company I have worked at, they have a team or working group that specializes in adherence to open source licenses specifically to avoid both the legal implications and the bad PR implications of misusing open source software.
