I'm skeptical of the 'federal law prohibits' angle. Money is fungible, and Massport revenue already supports a network of public parks, and it has historically owned transportation infrastructure like the Tobin. It owns streets in Southie and Eastie.
Aloisi's argument is that massport's surplus should in part be used to expand / improve transit access to the airport.
Massport is an artificial entity. Ultimately, all the money - including the money from developing land in Southie - belongs to the commonwealth. Its just happens that Massport revenues an easily-accessible pot that money that is separate from the general fund, so it makes it easy for polticians and advocates to frame artificial trade-offs between a constrained set of ivnestment options.