No, he is genuinely a Democrat. His voting record is left of center. For his first nine years in the Senate (2011 to 2019, inclusive) he has averaged 57.2% from Americans for Democratic Action. For his first ten years in the Senate he has averaged 26.5% from American Conservative Union. So he votes with liberals a little more than twice as often as he votes with conservatives.
Yes. The exact nuances of his voting record mean less than nothing, if you support the filibuster, you support Mitch McConnell having unchecked control over the Senate indefinitely
Or maybe Manchin believes that having the filibuster may be important for Democrats when Republicans retake majority control of the Senate again.