French counting numbers are also still partly 20-based, as are danish and welsh. It's said it's a remnant of an earlier common numerical system. It's then even possible that human languages came from a common origin with a 20-based system, and some kept it.
Danish counting is no more base 20 than English (that is, it is until 20). The words for 50, 60, 70, 80, and 90 are derived from an old word for twenty, but it's really no different than English having "twenty", "thirty" and "fifty" instead of "twoty", "threety" and "fivety". It's not like French where 99 is 80+19.
It's not similar in function. It's an etymological curiosity, and one that many Danish speakers are not aware of, we're certainly not doing multiplications in our head when we use them. The words are just words. The underlying structure in Danish is counting in tens. There's a word for each ten and you add 1-9 before it, but not so in French.
Monogenesis is pretty deeply out of favor in historical linguistics these days. Afaik most linguists specializing in far ancient protolanguages just frankly admit that their techniques aren't capable of deciding it in either direction.
There's nothing even near consensus on how far back you'd have to reach to figure it out either. With theories it could be as recent as the out of africa expansion 50k years ago, or emerging with or even predating emergence of anatomically modern humans ~300k y/o, or literally anywhere in between.
Confidently establishing linguistic monogenesis either true or false is like nobel prize shit with significant ramifications across the entire understanding of human language. They have definitely considered number bases.
The simple near universal human fact of "fingers + toes = 20" means any number of unrelated languages are likely to converge on that base and its factors. It doesn't disprove that theory it just isn't a useful bit of data towards it either way.
