How poor was our tally of objects that could produce potential extinction-level events back in 1998?
It was fairly poor.  Wikipedia has a list of large, potentially hazardous asteroids, and 9 on that list were known before 1998:

2201 Olijato
1620 Geographos
4183 Cuno
1981 Midas
3122 Florence
3200 Phaethon
4486 Mithra
4197 Toutatis
(4953) 1990 MU

The largest of these, 3200 Phaethon, is 5.8 km, so considerably smaller than the Yucatan impactor, but big enough to do plenty of damage if it does eventually get nudged into our orbit
At that time we hadn't observed (53319) 1999 JM8 a 7 km asteroid, so it is not inconceivable that we might not have observed a 10 km asteroid (similar in size to the low end of estimates of the Yucatan impactor).
Now Armageddon has a &quot;Texas sized asteroid&quot;, and Texas is huge, 1300 km from Brownsville to the Rita Blanca grasslands.  It is considerably bigger than Ceres. There is essentially no chance that an asteroid the size of Texas would have gone unnoticed by NASA, indeed it would probably have been a naked eye object and known by the ancient civilisations.