Did the Space Shuttle really wait until 50 feet above the ground before they put the gear down?
If you're looking for a definitive source, how about the Shuttle Crew Operations Manual. It's essentially the POH for the Space Shuttle. Secton 2.14-2 says:


  The landing gear is deployed at 300 ± 100 feet and at a maximum of 312 knots equivalent airspeed (KEAS).


Although, the targeted deploy speed was 288 KEAS according to the Normal Procedures section 5.4-7, which matches up with approximately what you see in HUD videos.

The manual says the gear may take up to 10 seconds to fully deploy, but based on HUD videos (measuring the time it takes for //GR// to change to GR-DN), this process pretty consistently took about 6 seconds and was completed by about 70-130 feet AGL depending on the deployment altitude.

The reason they deployed so late is because the shuttle was a very non-aerodynamic "airplane" to begin with, and lowering the gear would reduce its lift-to-drag ratio even further. However, there were non-standard procedures in place (see section 7.4-25) where, if the speed brake (a split rudder&nbsp;&ndash; not a wing surface) was stuck closed, they would have lowered the gear earlier in order to act as a speed brake. Note that the hydraulic system was incapable of raising the gear in-flight, so once down, it stays down.

Because the shuttle was a "glider" (a generous use of that term) at landing, with no chance of going around, the gear absolutely must work on the first try. A shuttle belly landing would have ended very bad due to its high speed and high angle of attack. In order to assure the gear would extend, there were several redundant systems in place:


The doors had a bungee-assist system which exerted 2000 pounds of force (~9 kN) on the nose wheel doors, and 5000 pounds (~22 kN) on each of the main wheel doors.
The nose wheel had a pyro-assist system which fired every time the gear was deployed, and helped assure it would lock into place.
The gear is normally deployed through a combination of "springs, hydraulic actuators, aerodynamic forces, and gravity."
However, if all else fails, and the gear does not begin to move within 1 second of issuing the command, a pyrotechnic initiator cuts the locks and forces the gear down.


So they were pretty confident it was going to work.

Interestingly, the original procedure for deciding when to deploy the gear was based on airspeed, not altitude. However, this resulted in an inconsistent safety margin. This quote from the Flight Procedures Handbook: Approach, Landing and Rollout explains it better (sorry, I don't have a link to a doc, I probably downloaded it from a nasaspaceflight.com forum a few years ago with a paid membership):


  The landing gear deploy cue for STS-1 through STS-4 occurred when the velocity decelerated through 270 KEAS. This corresponded to an altitude of 200 ft on the nominal energy trajectory. Flight STS-1 followed the velocity cue procedure which, due to its much higher than expected energy, did not occur until an altitude of 85 ft. Had the gear been deployed at 200 ft, some of that excess energy would have been dissipated. Flight STS-2 was very low on energy, reaching a maximum velocity of only 274 KEAS at 1100 ft altitude. The 270 KEAS cue occurred at 600 ft, but the actual deploy occurred at 400 ft, adding to the already existing low energy condition. Lower energy occurred since the nominal altitude for gear deploy would have occurred at 200 ft on a nominal trajectory. Flight STS-3 was high on energy, not decelerating through 270 KEAS until an altitude of 87 ft. T/D occurred earlier than expected on STS-3 and the gear was actually down and locked only a couple of seconds before first wheel contact. It was after STS-3 that altitude was selected as the gear deploy cue because it would compensate for off-nominal energy conditions, not make them worse, and still satisfy safety concerns. Downrange was also considered as a gear deploy cue, and it too, had advantages over velocity.


As far as the sign you are referring to, I see three possibilities:


They were referring to the very early missions which sometimes saw very late gear deployment,
They were rounding down for dramatic effect, or
They simply made it up on the spot as if it were a statistic.