Probably their high speed and great acceleration makes them suitable for such role.
Speed and acceleration are useful but hardly essential for SEAD, not many dedicated EW attackers were known for their speed or acceleration, and many, like the famous EA6 were not even supersonic.
What almost all dedicated EW aircraft were known for were their endurance and carrying capacity, features the J8II is not well endowned with.
You want good endurance because odds are the enemy are not going to be so obliging as to light their radars waiting to be ARMed, so you ideally want Wild Weseal birds overhead all the time so whenever a hostile radar lights up, you have assets on hand ready to take them out.
You want good carrying capacity because unless you go for a substantial airframe modification to embed all the dedicated EW systems in the aircraft itself, you need to be lugging big EW pods on top of your ARM complement. And that's just the bare minimum. Ideally you would also want a pair of short range missiles for self defence and also some drop tanks. The J8II simply lack the hardpoints needed to carry all those loads, it can barely manage the minimal pods plus ARM package.
I think the J8II would be fine carrying a pair of A2G stand off missiles and can fill a niche role as a quick response striker against high value targets that require an extra short kill cycle, so for example, if you equip the fast J8II with a pair of even faster CM400AKGs, you can drastically cut the kill cycle compared to a JH7A with KD88s for example. The Cm400 does have an ARM function, which may be where the wild Weseal claims came from, but I think the J8 upgrade would primarily be to give them stand off A2G capacity, with SEAD a possibility because of the missiles being integrated rather than because the J8 was being specifically modified for SEAD or DEAD roles.
In any case, if the PLAAF is giving J8IIs a ground attack capacity after all these years, I think the primary reason would be to give these still new airframes a purpose now that the J8II has been displaced from its traditional interceptor/air superiority role by the far more capable J10s and J11s rather than because the J8 is particularly suited to the role.