[2019] Got it on my alt. I guess I’m not logging in on my main till season 10 because my main is already tier 100 level 100

You will not get battle stars to start next season. You may see the message show up due to you not logging in today, but you will not gain any battle stars to start the new season.

The Reddit community does not encompass the entire mobile player base. Players who are here on reddit are typically extremely active and may have a higher tier then the average player. There are plenty of players who have not yet completed tier 100 of the Battle Pass.