First alleged fixed price and now additional costs - legal?

you are quite a clever guy. No, it doesn’t come to that.
For the OP it is indeed relevant that possible maximum depths for BP or similar are specified.
However, the contractor cannot claim additional costs solely based on the conditions on site if he has not inspected it beforehand.

Either I misunderstand something or you do.
You wrote: If the general contractor does not conduct an inspection, then he cannot request additional payment.
I wrote: This was already addressed 2 pages earlier, namely if the GC states in the standard GC contract that the fixed price applies under the lump sum for structural engineering (steel) and foundation (earthworks). If the lump sum is exceeded, then additional claims apply.
The conditions on site exactly influence this point from the contract. If the lump sum assumption (removing 30cm topsoil and soil quality is good) applies, then the lump sum holds and the fixed price is maintained; if there are deviations from the assumption and the lump sum, then there is an additional claim.

Is it stated like this in the construction contract? Hence also the statement that the GC has no risk at all with such standard fixed price offers with lump sums. And what counts is not the vendor’s talk but solely the contract signed by the OP.
That is just crap but also nothing new.
The salesperson did inspect, but he is not the site manager and in the worst case not even an employee of the GC but self-employed.

you are quite a clever guy. No, it doesn’t.
For the OP it is indeed relevant that possible maximum depths for BP or similar are specified.
However, the BT cannot claim additional costs solely due to local conditions if he has not inspected the site beforehand.

But he can. The construction service description usually specifies exactly what is included. For example, removing 30 cm of topsoil and storing it to the side. There are also phrases included that make additional work and additional costs due to local conditions billable.