Whoever gave you the ruling was mistaken.
According to level 4 judge Chris Richter, the Inkmoth Nexus will remain a 5/5 artifact creature land but will lose flying, infect, and its Blinkmoth subtype after the turn ends.
Some more details on what happens afterward:
When the turn ends, the animation effect from the Inkmoth Nexus will
end. When this happens, the Nexus will be become a 5/5 artifact
creature land. This Nexus will still have the normal abilities of an
Inkmoth Nexus, the mana ability and the ‘animation’ activated ability,
as well. But this permanent will not have flying or infect at this
point. This is because Inkmoth Nexus’ animation ability has a duration
that lasts until the end of turn, while the animation effect from
Tezzeret’s ability does not. The animation effect from Tezzeret’s
ability will last as long as that permanent remains in play. If no
other effects are applied, then this Nexus will look like this (a 5/5
artifact creature land with Nexus abilities) for the rest of the game
or as long as this Nexus remains in play.
However, you may just want to use additional animation effects on this
Nexus for strategic reasons. If the Inkmoth Nexus’ animation activated
ability resolves again, then this permanent will become an 1/1
artifact creature land with both flying and infect. It is a 1/1
creature again for the same reason as why it became a 5/5 earlier, the
power and toughness setting portion of the Nexus’ effect is applied in
timestamp order and overrides Tezzeret’s effect that makes it a 5/5.
And, just as earlier, you can use Tezzeret’s ability again as well.
Doing so will make this permanent into a 5/5 creature with both flying
and infect. Then, as before, at the end of turn when the Inkmoth
animation effect wears off it will become a 5/5 artifact creature
land.
