It's a curious sentence and one of only two to mention timing, the other being the introduction's mention of the "unpredictable timing" of garbage collection.

Obviously it is true in a single threaded system. If the one and only thread executes a basic block of instructions, none of which call into memory management, or call any functions which might do that, then it holds that it's not interrupted by garbage collection.

If there are threads, and a stop-the-world discipline for GC, then obviously not.

Note that garbage collection itself can have a switch, not involving disabling interrupts. Of course, you wouldn't want to insert that around every block that doesn't need GC. I'm just mentioning that in order to note that we don't a heavy hammer like disabling interrupts in order to momentarily disable garbage collection. When GC is disabled, it is still possible to allocate objects, but no garbage collection passes are invoked to reclaim storage. This means that if the allocator has exhausted the available heaps, since it is not able to run a garbage collection pass, it has to ask the system for more memory for a new heap.

I've used a switch like this around calls to a Yacc-generated parser, due to the Yacc stack being invisible to the garbage collector; newly allocated objects could be prematurely reclaimed as they are being "laundered" through the Yacc stack before being nailed into the abstract syntax tree.