I think that means the (emulated) machine is filled out to a full 64KiB. In the days of "4K BASIC", and 4KiB memory boards, a full 64KiB would have been impossibly expensive, but BASIC didn't care: The "Bytes Free" was how much room was left in RAM for what program you might care to load or toggle in after the BASIC interpreter had loaded.

Those 4K boards in the original Altair were expensive: $264 kit in 1975 [1].

I do recall the smell of those TTYs: Machine oil, bronze bearings, paper and ink.

Nice to see those 16 Address Bus switches, with the lower 8 switches doubling as data switches for the "Deposit" and "Deposit Next" switch.

edited for reference

[1] http://www.pc-history.org/altair.htm