No idea how others might evaluate this, but for me, placing the collectors in the soil is part of it. If soil needs to be removed for this purpose, it must also be refilled. The note "installed" in my opinion includes a complete service. Your chances are not that bad, provided there are no further agreements, which are not mentioned here.
If you cannot come to an agreement, I would involve a specialist lawyer. However, at the same time, I would postpone this dispute as far back as possible.