Debatable. Not theologically settled.
Actually, in the Roman Rite, it's almost morally certain that it's invalid ... due to the way the essential form is worded.
COMPLETE IN YOUR PRIEST the summit of your ministry.
"Comple in Sacerdote tuo ministerii tui summam, et ornamentis totius glorificationis instructum coelestis unguenti rore santifica
Bassically it assumes this is a priest (if he weren't the essential form would be incorrectly designating the matter), and it refers to COMPLETING something that's already there, and not merely granting it.
What was held AS A MINORITY OPINION was whether Episcopal Consecration can be validly conferred on someone who isn't a priest, in general, and there may be some Eastern Rites that are not worded as above, the the attempt to perform the Latin Rite consecration on a non-priest would certaily be invalid.
So, it's not really debatable ... except in general terms.
Nothing exposes someone more than when they're soundly refuted about something and so they come up with a backup reason, and that's the surest indication he's already made up his mind, out of spite, to continue slandering Archbishhop Thuc, but wants to make certain exception (+Lefebvre line) to the crap theology of "internal intention" he invented, where some occult Freemason could "wish away" the Church intention for the Sacrament.
+Lienart's having been a Freemason was proven, and not just speculation, and +Lefebvre admitted it on two different occasions. If one were to believe Scuмmage's fake made-up theology, there's an extraordinariliy high chance the +Lefebvre was never a valid priest, and therefore we've had millions of invalid Sacraments among Traditionalists. Except of course that this buffoon simply made it up.