Although the python lib is undoubtedly outdated, this is not the cause this time. It depends on the server you are requesting:

Steemit node:

s = Steem(nodes=["https://api.steemit.com"])
print(s.steemd.get_account_count())

3619629

My node:

s = Steem(nodes=["https://api.moecki.online"])
print(s.steemd.get_account_count())

1894124

However, this is no less worrying because we can now ask ourselves why the steemit node throws out such an obviously erroneous number.