It’s not true. Inline functions still exist in bytecode, so you cannot mark function as inline and decrease methods count
Impact of methods count on performance is very limited and hardly measurable. Dex size is important for application startup time, but it’s not only about methods count. Also inline functions will increase dex size, not decrease, especially if you inline functions with big body.
And inline functions is not something related only to functional programming, so even with other inline functions advantages it doesn’t look like argument for functional programming