I say add it. We know from the Golden Century book that the Clans developed their own Mechs and designs, so why not add it? In a small Clan like the Scorpions the Sun Spider could literally be a few Stars strong in its totality, hence it rarely being seen by other Clans, and when they see it they go "Meh, do not care, we have got Summoners and Timberwolves, you keep your little toy" and never bothered trialling for it. Or if they did recover one, they made a point of scrapping it to slight the Scorpions.
Other Clans could have their own 'in house' designs, sure they are rare as hens teeth and could be the providence of the smaller Clans or those who didn't get involved in REVIVAL, hence them not being seen by the Inner Sphere until the Post Tukkayid or even post Bulldog/Serpent period.
So if there's ever more of these, I say go for it. Lore gets re-written and updated and retconned all the time, so there's no point in sitting and going MUH CANONS!!!! or screaming and clawing at your hair whilst vomiting in hysterical rage or any such nonsense. Just shrug, and go "Well, more Mechs, more lore, yum!"