They're running Bravo. That's a fun editor. An improvement on most of its successors.
You edit both programs and documents in Bravo. Programs in Mesa are usually written in variable-width fonts. (Somehow this never caught on. It looks nice. I used to bold Mesa keywords.) The file format is plain text, then a control-Z, then formatting information. The compiler reads up to the control-Z and stops, thus ignoring the formatting.
It was more of a "tongue in cheek" comment about how fair they have progressed with the restoration with how little time they have spent on it (as they are not restoring this as a full time job).
In other words they are progressing so well they will have python running on it despite the "limitations" of the Alto just to prove they can even if its just a simple hello world. I'm not actually expecting them to port python to it.
I think this is more likely. In collecting older computers I've often come across "complete" systems which were cobbled together from a bunch of disparate systems of different origins.
Generally this isn't a problem is the person putting together the system is actually running it, they might do this to keep it running after its maintenance contract has expired (happened with a lot of VAXes and PDP-11's) but it is a problem if a scrapper wants to get a premium for a "complete" system and they don't know if it works or not so they just put "the required boards/parts" together and call it a system.
This also happens to cars. There are two Hungarian men, siblings, who have invented entire cars out of thin air cobbled together from whatever pairs. They have been 'establishment' for many decades so it's incredibly hard to undo the damage they wrought based on documents from corresponding age found in archives.
It seems weird that Alan Kay would have an Alto that is just a jumble of incompatible parts. And that YC bought a random mouse on ebay and didn't tell these guys about it.
In the early 90s, on two separate occasions, I was offered
"3 Altos" for just a few dollars (one batch was from Stanford that had been using them for printer servers, and I don't remember where the second batch came from).
Over the years I wound up donating them to various places (the Computer History Museum, the Kyoto Prize museum, the Boston Science Museum, etc.). We kept one at Viewpoints as a mascot, and my last one I gave to Sam Altman because he wanted to do a restoration. That one turned out to be a late hybrid that Ken Shirriff and company are having fun puzzling out.
The "random mouse on ebay" was at my suggestion, because the Alto I donated to Sam didn't have a mouse.
Most of the original runs of Alto IIs (at least 1500 of them were built) cost in 70s dollars about $22.5K apiece. Inflation (according to the web) since then has been about a factor of 5, so each Alto would have been about $112.5K (and I'd love to have a $112.5K personal computer today -- that would really be "computing in the future" as we did at Parc).
Actually it doesn't seem that weird to me. I don't recall anyone who bought a system from Xerox (as I recall they were in house development only and cost about $75,000 each to produce). But I do remember when they were being scrapped and the word went out that some ex-Xerox employees were rescuing them and looking for others to take some. I was sorely tempted to take a Dandelion machine (this was the mid-range D-machine that I was most familiar with). I could easily imagine that Alan said "I'll take one" and whomever was organizing the rescue made sure all of the "parts" were included when it was long past the time when you could easily boot one up and see if it worked or not.
You edit both programs and documents in Bravo. Programs in Mesa are usually written in variable-width fonts. (Somehow this never caught on. It looks nice. I used to bold Mesa keywords.) The file format is plain text, then a control-Z, then formatting information. The compiler reads up to the control-Z and stops, thus ignoring the formatting.