That's what they're speculating, that's speculation only.
( ( That ( 's ( what ( they ( 're speculating ) ) ) ) ) ( , ( that ( ( ( 's speculation ) only ) . ) ) ) )
(ROOT (S (S (NP (DT That)) (VP (VBZ 's) (SBAR (WHNP (WP what)) (S (NP (PRP they)) (VP (VBP 're) (VP (VBG speculating))))))) (, ,) (NP (DT that)) (VP (VBZ 's) (NP (NN speculation)) (ADVP (RB only))) (. .)))
That is just speculation.
( That ( ( ( is just ) speculation ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ is) (ADVP (RB just)) (NP (NN speculation))) (. .)))