I don't know about the anti-MS motivation.

When MS changed from 16-Bit DOS to 32-Bit Windows [95] the then current nmake 1.4 (included in MSVC 1.5) exposed a massive bug in the new 32-Bit environment.

As a consequence MS published a bugfix (KB132084) which offered the fix (nmake 1.5) as a freely available download for that program, which was otherwise only available as part of the commercial MSVC.

Also in later years nmake was only available as part of the payware compiler. When did MS start to give compilers away for free (aka Express editions ?)

So at the time of nmake 1.5 the (KB) knowledgebase page of MS with its few KB(kilobytes) download of a free nmake was a godsend for the Perl people, who wanted to ease the life of Windows users.

Update: make explicit KB = (ms) knowledge base and KB = K byte