That was in relation to the concept of a BR6 being added to the game, allowing the BR’s to be decompressed.
BR3.5 in this case would be the new BR4, the old BR4 becoming BR5, and the old BR5 becoming BR6. This would help a lot for those weapons which are sad and underpowered in BR4, but too powerful for BR3, and vice versa. So you would get a new BR4 with the likes of the Jagdpanzer, KV-1 and maybe the Chi-to.