False. The source code for DOOM, and thus Doom Legacy, and thus SRB2, is released under the
GNU General Public License v2, as listed at the top of every source code file. This grants users the right to modify, distribute, and sell the source code (and compiled executables)
only. The condition is that the derived source code must be released similarly, under the same license or a later version thereof, and with public access to said source code available. (Now, original source files that were made specifically for SRB2 may not be licensed under this same license. You'd have to check for all of that. Also note that some dependencies required to build SRB2 may also not be released similarly. Check these things thoroughly if you intend to create a new project off of the codebase,
especially if you intend to sell your results.)
This license does not apply to any asset files, since they were created independently of DOOM's original IWAD (which in itself can't be redistributed legally outside of purchasing the game itself anyway). The asset files themselves are the sole property of the people who made it, and since they've been released under no such license, they may not be used in other projects. Now, it's been basically accepted that repackaging the assets or modified versions thereof in addon WADs for SRB2 is allowed, and there's generally not much that the team does unless original content is blatantly stolen and used in other games, but don't mistake this for being able to use a modified srb2.srb in a commercial project.
Basically, if you use the source code as the base for another project, whether commercial or not, it must be with a completely original IWAD. (But why would you want to use the engine for anything else? There are many much more modern engines out there to use already.)
Disclaimer: I am obviously not a lawyer and may be wrong about things.