They ARE dimmable, I've done it several times, but not in an instrument lighting situation, more like just getting them at proper intensity.
You simply have to understand that the dimming is varied resistance, when you change it you are changing resistance using a variable resistor. Or what a dimmer is.
I'd bet that linked $34 part could be replaced by something from Digi-Key for like $2-$3, or, another whoppingly overpriced part. I see 3 wires and no PWM occurring there at all, or a simple variable potentiometer. Can't tell without breaking it apart to see if it's chipped or not. Thinking you need more than 3 wires to do a chip. I use like a cheap $3 potentiometer to test/set the volts to where I like, pull meter and measure the value on it with voltmeter then add that value of resistance on the power lead with simple resistors that cost maybe ten cents each.
LEDs may not like going down close to dead zero light, I can't speak to that. ALL LEDs dim using varying amounts of resistance, usually from 300-680 ohm for 12 volt (red or yellow, blue may be different) to get normal light with max life. More light=less life. Most are limited to like 3-5 volts input with no resistance, you add ohms to use higher voltages. Get them far cheaper at like electronics place rather than car parts, you'll pay WAY too much getting them there.
Problem is the car dimmer may be in the wrong range of resistance needed there. Another issue is that LEDs have pretty focused narrow beam light output, they only light say over a small arc of viewing angle, not like incandescent which can light across a maybe 270+ degrees arc. Why you see all the LEDS mounted in taillight bulbs at weird angles to complement each other as well as for the added light output. On instruments that could easily result in too much light on part of the gauge and not enough on the rest of it, like if one bulb is out. Of course you could simply add more than the factory used bulbs for. They are easy to drill for and mount.
The later Ford PCM cars have so much trouble emanating from radiator cooling fan issues that on all 3 of my zetec cars I have LED panels made to indicate when a/c compressor clutch, low fan, and high fan each cycle on, it is super helpful in knowing quickly what can be wrong with the cooling system in the cars since so many issues come from electrical failing there somewhere. The most common issue failing the cars to put them in the junkyard is cooling problems. Keep the electrics working correctly and the engines cannot be blown up, they routinely go 300K miles. One severe overheat though and kiss all that goodbye. The cooling fan resistor on them has a tendency to fail with little warning. You can quickly deduce a/c issues too using that light.
Done right with correct resistance used LEDs last forever, very rare to fail one and ideal for instrument lighting if you get it worked out. Use quality parts, the cheaper Chinese ones can drop in light output quite a bit after only a small amount of time. I've seen blue drop 50% in 2 months. And get the polarity clear in your head before applying power, even one time power applied backwards can fry some almost instantly, even though they ARE diodes and should stop power the other way, it is intended to be only a small amount backwards.