One could say there is more danger in going too hard on a 20-min Tempo Run (and positive splitting or having form break down) than going too Easy. You go too Easy and you might hit "Uptempo" (aka closer to Marathon Pace). It is still a good "high end" aerobic stimulus....albeit probably too short for most marathoners (who may very well do 2 x 20min with a 5-min rest or something).
Now of course if you go too hard and cross the Threshold you are still getting benefits! You cross the threshold in Vo2max repeats/intervals and they are of course harder. You race a 3km/5km/10km and the pace is faster and it is harder. Good benefits in moderation.
But the context of the "20-min Tempo Run" as a workout alone is it is a great workout to slowly and consistently build fitness and stamina early on in a "aerobic base phase of training" (as well as in the Taper phase to not overtrain/over-strain). It is also a good "moderate workout" to have in a mix between the very hard and faster interval sessions (think Vo2max repeats like 6 x 1km etc.) and other workouts like hard Long Runs. A type of workout that could be done nearly every week out of the year.
Let's now consider a "timed-interval" session (I call it "Fartlek" although it is certainly not spontaneous like the origin of "speed play") session of 10 x(1-min hard, 1-min Easy). It is only a 20-min workout.
Now this workout can be as hard or as easy as we want it too be. We could run the 1-min hard sessions at faster than 5km pace and then "float" the recovery at a steady pace. In that case we could induce heart-rate spikes into the "Vo2max training" realm as we are certainly hitting velocity at Vo2max. We could be inducing a lot of lactic acid and have our form break down if we push really hard.
OR we could do the 1-min sessions at just around "only" 5km-10km pace and take a very easy 1-min on the jog sessions. In that scenario it is a much more moderate type of workout even though we are getting the legs used' faster running' at 5km/10km pace.
So in both cases the workout is 20-min long. If we average the HR data for the 20-min maybe it was around 85-90% (much like a straight-up 20-min steady, even paced "Tempo Run)....but maybe it was only 80% (on the easier side). So we induced a very different stimulus. We crossed the threshold and spiked the HR over 90% on the hard minutes, we got the legs moving faster than 5km pace etc.
Heck we could even do this workout as a session of 10 x 300m with a 1-min float/jog (more formal). What is it labeled as then? Well it depends exactly on the pace/intensity of the fast running and the pace/intensity of the recovery.
The point is we can choose how much lactate we want to induce. You want to always know the purpose of the workout in the context of the overall training plan and how it fits your racing/performance goals. No doubt you can get in shape trying to blast a 20-min "hard run" at 10km pace (faster than true LT), but then is that simply a time-trial race to see how long you can hold 10km pace?
Bottom line: Running Economy is the real key we want to develop. Throw Vo2max numbers out the window. That is the end goal. Running Economy is reflected of course in our Velocity-Pace at LT and our Velocity-Pace at Vo2max....which will be directly reflected in actual race performances from 3km-marathon. The Training and workouts we do are all to support this development of efficiency. In simple terms it obviously takes a mix of high mileage, LT workouts, Speed Training (be they "Reps" or Vo2max intervals. We train the systems at the neuromuscular level to improve form and mechanics, we strengthen the tendons and muscles, and we build mitochondria size/density and capillary beds and stroke volume (to name a few adaptations) etc.
On a final note check out some of the old "Lactate Shuttle" threads and research if you want to get more into the science on LT training specifically.