"Whoever can be trusted with very little can also be trusted with much"
I believe the parent is saying that if Keysight was lazy, shortsighted, or irresponsible with these historical documents, they probably won't treat the design, construction, and your purchase of an expensive oscilloscope with a different attitude. So, ostensibly, buy Tektronix (or, for hackers, buy a Rigol - more value for less money; I love my $900 MSO1074Z-S and the closest Keysight device is a $8000 oscilloscope that doesn't include most of the software decoders you want, and doesn't include the $3000 function generator).
But I do remember in 1994 or thereabouts, calling the h-p customer service number with questions about the old hp 'scope I had just picked up for $50 that came without a manual. The lady who answered the phone put me on hold to go digging through file cabinets and then came back with the documentation to politely answer all my questions.
I have never been anything but completely impressed by all of Hewlett-Packard's instruments or any interaction I've ever had with them.
And BTW, IIRC that Rigol made some of HP's low end DSO's back in the early 00's.