Freed-up capacity is freed-up capacity, no matter whether it was created by building an additional lane or by motivating some people to switch to alternative modes of transport. If enough people switch to public transport to noticeable reduce congestion, the reduced congestion makes driving more attractive again, so some people will switch back to driving, or people who used to drive anyway will now make longer trips, etc.

So if you're just building out transit while keeping road capacity the same (less common with dedicated bus lanes or light rail, because space for those is often created by converting former car lanes, but with "other off-grade transit" modes you need to make a conscious effort of reducing road capacity if reduced car usage is your goal), it's entirely possible that public transport use increases, while passenger miles travelled by car remain the roughly same long-term.