"Over them both loomed a giant in armor made of stone, but when he opened his visor, there was nothing inside but darkness and thick black blood."
Compared with Robert Strong:
“The silent giant.” Lord Randyll grimaced.
(He's called 'the silent giant' and is 8ft tall)
"His armour was plate steel, enameled white"
(Never takes off his armour, it's huge and very distinctive, specially made)
“The crest atop it was a stone fist”
(An allusion to Gregor, he wore 'stone' on his armour)
“long parapets built into the very stone of the mountains”
(Mountains are made of stone, this is a cryptic nod that it's formerly Gregor)
“Whatever the face hidden behind Strong’s helm, it must remain hidden for now.”
('opening visor' is something Strong hasn't done yet, and we know he's hiding something)
“I am sending Balon Swann to Sunspear, to deliver him the head of Gregor Clegane.”
(Whether that head was real or not, I think we suspect there's no head on RStong)
“But this venom has been thickened somehow, so as to draw out the Mountain’s dying.”
“Thickened? Thickened how?”
(The dream insinuates the giant has thick black blood - RV thickened the black poison he put into Gregor's blood. The poison seemed to spread and turned his insides black)
“When Oberyn spun the shaft between the palms of his hand, they glistened black. Oil? Or poison?”
(Again, the poison in Gregor's bloodstream was black)
“Be that as it may, his veins have turned black from head to heel.”
(RStrong's blood seems to have been turned black by the venom. RS has black poison for blood)
Overall, a giant armoured in stone with no head and thick black blood fits Robert Strong perfectly.
RE 'it's too obvious' ... well it wasnt obvious for a long time, this is from GoT.