It's possible your colleague misremembered or conflated two names. After all, the prefix Micro- is common enough in computer-related branding (see also the MicroBee, Microtan 65, and others).
It's also possible your colleague meant the MicroAce, which was an actual clone of the ZX80. This one was probably made well-known by the court case around the software copyright. Sinclair and the makers of MicroAce ended up settling out of court, and MicroAce carried on selling in the US, separately from the Timex Sinclair 1000, which was a licensed ZX81 clone.
Was this a real machine and, if so, how did it differ from the other two [, being the ZX80 and ZX81]?
It was ZX80 compatible, so a little different from the Timex Sinclair 1000, which was ZX81 compatible. Here in the UK, Sinclair stopped selling the ZX80 after the ZX81 was introduced to the market. But in the US, both the MicroAce and the TS1000 kept on selling contemporaneously (not so surprising, since they were being sold by two different, independent companies).
There is not much difference between the ZX80 and the MicroAce. The only difference is that the MicroAce can be upgraded to a whole 2 kilobytes inside the case. Oh, and the ROM is different as well, but that's just because two of the data lines have been swapped over. So you can't just drop in a ROM from a ZX80 and have it work. But this change is so trivial. Not enough of a difference to convince the judge of that court case.