What's the history of valve adjustment on the vehicle? They should be adjusted every 15k miles.
It seems very reasonable that they want to adjust the valves before doing the compression test. It's a sign they are mechanically astute about the vehicle.
Did it start any better after it got new glow plugs?
It's possible that the compression is low with that amount of miles, but it is also possible that a combination of bad glow plugs and poorly adjusted valves was causing the problem.
I don't see any way to know that the answer is without the test.
You could just get the valves adjusted and see what happens. It it starts ok then, you could avoid the compression test and assume the compression is acceptable.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13