That is mentioned in the article, although could perhaps be more emphasized since it does mean the "obvious" fix is not possible:

Commonly available protected 18650 cells don't fit in the badge's cell holders because they are slightly longer.

FWIW, I have battery holders that deathgrip unprotected cells, so even finding a proper holder for those is a crapshoot.

I did indeed totally miss that, thanks for pointing it out!