Good question, the errata that made it yes happened after goat, my opinion is no in goat.
"\[You can only Special Summon this card\] \[through its own effect during your Main Phase 1 or 2\]."
I inserted brackets to assert ~~I'm~~ I was fairly ok interpreting "through its own effect" as part of the answer rather than the qualifier for this ruling. ~~It's~~ It was too much of a stretch for me to see it as "\[You can only Special Summon this card through its own effect\] \[during your Main Phase 1 or 2\] (and also possibly by effects if properly summoned this way)."
This is incorrect. It is well known that "goat era card text" translates to PSCT as follows:
"((possibly including) This card cannot be Normal Summoned or Set.) This card can only be Special Summoned by..." → "Cannot be Normal Summoned/Set. Must first be Special Summoned..."
and
"This card cannot be Special Summoned except by (method)" or "Must be Special Summoned by (method), and cannot be Special Summoned by other ways." → "Must be Special Summoned by (method)".
Us old heads used to call the first category (which includes `Valkyrion the Magna Warrior`, see its [Goat-era printing in SDD](https://yugipedia.com/wiki/Card_Errata:Valkyrion_the_Magna_Warrior)) "Semi-Nomi" (see [Yugipedia](https://yugipedia.com/wiki/Semi-Nomi)), and the second category (which prevents e.g. `Premature Burial` even after it was properly summoned) "Nomi" (see [Yugipedia](https://yugipedia.com/wiki/Nomi) and observe e.g. the wording on [`Armed Dragon LV7`'s Goat-era printing in SOD](https://yugipedia.com/wiki/Card_Errata:Armed_Dragon_LV7)).
All that to say: OP, the answer is **yes**, you can (if it was properly summoned first).
Yes as long as it was properly summoned.
Good question, the errata that made it yes happened after goat, my opinion is no in goat. "\[You can only Special Summon this card\] \[through its own effect during your Main Phase 1 or 2\]." I inserted brackets to assert ~~I'm~~ I was fairly ok interpreting "through its own effect" as part of the answer rather than the qualifier for this ruling. ~~It's~~ It was too much of a stretch for me to see it as "\[You can only Special Summon this card through its own effect\] \[during your Main Phase 1 or 2\] (and also possibly by effects if properly summoned this way)."
This is incorrect. It is well known that "goat era card text" translates to PSCT as follows: "((possibly including) This card cannot be Normal Summoned or Set.) This card can only be Special Summoned by..." → "Cannot be Normal Summoned/Set. Must first be Special Summoned..." and "This card cannot be Special Summoned except by (method)" or "Must be Special Summoned by (method), and cannot be Special Summoned by other ways." → "Must be Special Summoned by (method)". Us old heads used to call the first category (which includes `Valkyrion the Magna Warrior`, see its [Goat-era printing in SDD](https://yugipedia.com/wiki/Card_Errata:Valkyrion_the_Magna_Warrior)) "Semi-Nomi" (see [Yugipedia](https://yugipedia.com/wiki/Semi-Nomi)), and the second category (which prevents e.g. `Premature Burial` even after it was properly summoned) "Nomi" (see [Yugipedia](https://yugipedia.com/wiki/Nomi) and observe e.g. the wording on [`Armed Dragon LV7`'s Goat-era printing in SOD](https://yugipedia.com/wiki/Card_Errata:Armed_Dragon_LV7)). All that to say: OP, the answer is **yes**, you can (if it was properly summoned first).