--listen --tail with a large number only getting a small number of messages

[BrenBarn]

I'm using a command like this:

matrix-commander \
   --import-keys ../element-keys.txt "secret password" \
   --room '!blahblah:matrix.org' \
   --tail 1000000 \
    --listen tail \
    --listen-self \
    --output json > chatlog.json

When I do this, I get about 1000 messages reaching back about a month. But I know that the room has far more messages than that. Increasing the value given to --tail doesn't change anything?

Am I doing something wrong? Is there some other way to say "get all the messages all the way back to room creation"?