how can low watt tube amps drive speakers with higher power requirements

If you push an amp into clipping trying to eek out more volume from an undersized amp then you can damage a speaker. A monster sized amp means it’s still just barely ticking over at volume likely to make your ears bleed. You’re more likely to damage your hearing than your speakers with too much amp.

There are SPL meter apps you can download for your phone. Check how loud your typical listening level is. Add 10dB for dynamic headroom and calculate how much amp you need. There are calculators online to help you compensate for distance and room reinforcement. Be careful believing published specs. Sometimes they are optimistic. Also there can be adjustment for impedance and whether the spec is given as 1W or 2.83V (1W @ 8 ohm).

"Back in the day" we bought stereo systems and the first thing asked was, " How many watts?" Every manufacturer played the numbers game to perfection because we had become "spec sheet scientists". {RMS @ 8 Ohms 20 - 20000...THD 0.000001 S/N ratio. Wow and Flutter. You know the drill} Whatever....we used to kick car tires too. Tonight my kit blasts out a pathetic 20 watts per channel. It runs fairly high distortion levels relative to most. I only have two channels. Two speakers. No sub. One turntable. One CD player. Simple, endlessly astounding, intimate, a sound stage to die for, with totally lifelike audio reproduction, (IMO) I live in my happy place. Why did I take such a long route to achieve such an elusive yet rewarding destination? I think because I wasn't "listening" , I was measuring, counting, and competing. Just sayin' (Wish someone had told me that a long time ago)

No you are not missing something.
Any amp will work. It’s just a matter of how well and the results will vary widely.

The main thing always is to avoid clipping. That is public enemy #1.
https://en.m.wikipedia.org/wiki/Clipping_(signal_processing)
Tube amps typically soft clip as do some others.  You seemingly get away with fewer watts when soft clipping but the result is distortion nonetheless.

My 91db speakers are driven just fine by my 20W tube amp.  (I will say, though, that with a similarly-priced SS amp at 40W, they sounded like sh*t.). I doubt that there's a general rule that you could follow that would trump the judgment:  "What does it sound like?  Do I like what it sounds like?"

If you buy a lot of equipment over the years you will find out that consumer grade receivers at 100 wpc have trouble keeping up with 25 wpc tube amps. You'll also find some 60 watt amps that kill 100 wpc receivers. The difference is power supplies. Good power supplies provide great experiences. The best way to stay on course is keep away from consumer electronics and buy quality audio from know good specialty manufacturers.