Splitting #6 strands to fit on two #10 lugs
No, it's illegal to split wires like that. Aside from failing on 110.12 "neat and workmanlike", it is also paralleling (not allowed without special apparatus) and 110.3b "using a product inconsistent with it labeling/instructions".
You need a lug connector rated for #6. Looks like an Alumiconn but is rated for #6. This is readily available at any proper electrical supply, Home Depot won't have it of course.
Many mistakes in this box.
Whoa! You can't hang both the appliance neutral and ground onto the neutral wire!
You have a 4-wire connection back to the panel, noting the big Al ground wire. You cannot combine neutral and ground anywhere (except for the N-G equipotential bond in the main panel). So that neutral-neutral-ground 3-way splice is for the birds, and must go.
My advice would be to buy a $4 accessory ground bar that takes #6 wire (and is itself aluminum, therefore compatible with both Cu and Al wire). Saw off all but 3 voids and a mounting lug, mount the mounting lug to the #10-32 screw hole there in the junction box, and land the two ground wires on two voids. Solved!
Box is too small
Follow ThreePhaseEel's advice on the 6x6x4 box (144 c.i.) which will actually be comfortable! If you can't remove the old box, stack a 4x4 extension box on top of it - looks like this box but with an open back. I do not use knockout holes on extension boxes.
The FMC conduit needs a proper fitting.
The wires shouldn't be marked with tape those colors, unless your hookup is 3-phase "delta" power. Also you are not allowed to re-mark hots to be neutral. Neutral must be the wire natively colored white. The remaining wires can be red, black or blue; those are all legit hot colors.
That box needs to be bolted down to something
Not flopping around.
Don't use wire nuts here, and never tape wire nuts.
Wire nuts are not reliable with aluminum wire. These should be appropriate lug connectors (i.e. Alumiconns, but sized for #6 wire).
Never tape a wire nut. Refuse to tape them, then give the nuts a "pull test" - holding the nut and pulling each wire in turn. Failing the pull test means a poor connection that will arc-fault. Taping it hides this important warning sign.
Do tape nuts that are holding a single wire; they're not designed for that and don't work well there.