Then they wrote the BASIC interpreter on the emulator, copied it to punched paper tape and Allen wrote the loader for that while he was on the plane to demo it for Altair.
On the real Altair 8800 hardware which they had never seen, it worked first time (they must have had some advance knowledge of the Altair’s I/O functions)
It was a series of amazing feats
https://en.m.wikipedia.org/wiki/Altair_BASIC
Edit: maybe this feat was not so famous because the Altair was not sold in numbers like the Apple II, which Steve Wozniak is well known for (25000 Altairs sold vs Apple’s 6 million - plus clones)