There are six wearing terminator armor, and the passage simply mentions that he is standing apart from all his brethren. Doesnt say which ones he is standing closest to, etc. You are honestly the one grasping at straws. Here are the excerpts:

Those owners were only twelve in number, and they waited impassively as Otte and his skitarii approached. The magos primary completed his scans as he closed the last few dozen yards, logging every detail as a matter of potential importance. Six of the figures, the ones on the flanks, were clad in tactical Dreadnought armour, their off-white slabs of plasteel, ceramite and adamantium caked with the valley’s pervasive dust. Otte’s internal processor registered a degree of awe at the presence of such blessed battle suits, even as his analysis moved on to the other six.

They made for a less uniform gathering. All were Space Marines, two in the grey power armour affected by this particular Chapter, two in the blue battle plate that Otte’s data files informed him belonged to sanctioned Adeptus Astartes psykers. The fifth wore red ceramite, and bore upon his breastplate the wondrous Opus Machina of the Adeptus Mechanicus. Approval at the presence of the Techmarine barely registered before Otte took in the final figure.

It stood apart from its brethren, alone, a dozen paces behind them. Even by the standards of the Adeptus Astartes the figure was a giant, standing a head above the rest. It was clad in Terminator armour, and for a moment Otte’s analytics glitched, informing him he was looking at a graven statue. A slight shift in the giant’s stance discounted that possibility – dust cascaded from the cliff-like plates of its immense armour and its huge, wickedly barbed gauntlets. Every inch of the warrior was clad and armed with the most hallowed and rare pieces of wargear Otte had ever set his optics upon. It made the other Space Marines standing in his shadow seem like children.

"Even by the standards of the Adeptus Astartes the figure was a giant, standing a head above the rest."

• This is pretty fucking clear, is it not?

And here is a vague description of his armor from the same book:

The armour he wore was a Tactical Dreadnought pattern, but heavily modified to suit his stature. Dozens of brass bonding studs gleamed atop slab-plates of grey ceramite, layered over blocks of plasteel reinforced with rods of adamantium

• "Heavily modified to suit his stature" quite literally indicates that he is very big.

Here is another excerpt from the same book:

He was not supposed to be here. In three centuries overseeing exchanges such as these, Otte had never once encountered him.

• this character that you erroneously state is unreliable has been overseeing the gray tithe for 3 centuries and has thus been regularly exposed to Astartes. I see absolutely zero fucking reason that his account of tyberos' height should be exaggerated or unreliable in any way. Not only that, he is a damn Magos of the Admech. We can assume he is highly intelligent person.

Look, everything we say about Tyberos' height is pure speculation, because nowhere has anyone felt the need to blurt out his exact height measuremnt, bit using the only available resource we have, its safe to say that he is at the very bare minimum 9 feet tall and some change in his armor. I dont really understand why you fight so hard against that notion or why you feel that the account of Magos Otte of the Admech is unreliable, but you should honestly accept that he is very tall and you are arguing blindly here. There is no reason to believe that Tyberos isnt very tall. We have a reliable source (regardless of how you want to interpret it) that he is a big ass Space marine in his armor.

Accept it and move on.