Lithuanian is very conservative in some aspects, and very progressive in others. For example, it has lost the neutral gender and it has innovated some cases that didn't exist in PIE. There is no scientific, consistent way to call it as a whole more archaic than any other language.
I’ve heard this and also heard it’s mostly complete rubbish. It might technically be the closest but that is still not close at all, simplifying like this doesn’t really help anything.
