one could provably use 2SAT to solve 3SAT problems, ie a reduction, its just that there is not a P-time/space reduction unless P=NP. in other words, converting 3SAT to 2SAT, or reducing 3SAT instance $A$ to 2SAT instance $B$, is indeed possible (using extra 2SAT variables and clauses!), but all known techniques lead to an "exponential blowup" in the size of $B$ wrt $A$ (ie space which also implies time). this shows how important it is to measure time/space requirements of a reduction and in fact there are many different ways to measure the complexity of a reduction. P-Time reductions are just more basic/common but many more complex "reduction complexities" are studied. so in this case in fact a P-time reduction from 3SAT to 2SAT exists iff P=NP...