Sean Manaea hasn't been quite as strong as his 3.07 ERA this season overall, but his ERA the first time through the order is an excellent 1.35. Equally important, the Cardinals have been terrible against lefties this season, with a 67 wRC+ as a team.
There's a little bit more risk on the Mets side against Gibson, who's also due for some ERA regression in the near future. However, given the eight-run total and the lack of a threat on the other side, this is more than worth paying the limited juice.