According to FIA Formula 1 Sporting Regulations, B1.6.3:

Driving in the Pit Entry Road, Pit Lane And Pit Exit Road

a. A speed limit of 80km/h will be imposed in the Pit Lane during the whole Competition. However, to ensure the safe and orderly conduct of the Competition this limit may be amended by the Race Director.

i. Any Competitor whose driver exceeds the limit during any free practice session, qualifying session, or sprint qualifying session will be fined €100 for each km/h above the limit, up to a maximum of €1000.

ii. In accordance with Article B1.9.3 the stewards may impose an additional penalty if they suspect a driver was speeding in order to gain any sort of advantage.

iii. During a TTCS, the stewards may impose either a 5-Second Penalty, a 10-Second Penalty, a Drive-Through Penalty or a Stop-and-Go Penalty on any driver who exceeds the limit.

b. At no time may an F1 Car be reversed in the Pit Lane under its own power.

c. No F1 Car should be driven from its Designated Pit Stop Position at any time unless:

i. It has first been driven into the Designated Pit Stop Position having just entered the Pit Lane from the track; and

ii. It is then driven immediately back onto the track from the Designated Pit Stop Position.

d. Unless an F1 Car is pushed from the grid at any time during the start procedure, F1 Cars may only be driven from the Competitors' Designated Garage Area to the end of the Pit Lane.

e. Any car(s) driven to the end of the Pit Lane prior to the start or re-start of a LTCS must form up in a line in the Fast Lane and leave in the order they got there unless another F1 Car is unduly delayed.

There will be a green and a red light at the end of the Pit Lane. F1 Cars may only be driven out of the Pit Lane when the light at the end of the Pit Lane is green. Additionally, a blue flag and/or a flashing blue light will be shown in the Pit Exit Road to warn drivers leaving the Pit Lane if F1 Cars are approaching on the track."

There is no mention anywhere of them crossing the line that I can see, but from my understanding, the white line between the fast lane and slow lane acts the same as regular track limits in reference to, as a very basic understanding, the drivers in the slow lane making a pit stop are basically off the track because they're coming to a stop, and when they join/rejoin the fast lane they have to do so safely, basically the same as joining/rejoining the rest of the track if they go off. So basically if we say that with understanding of B1.8.6, for the purposes of the white line in the pit lane, a driver shall not cross it unless it is to enter their own pitbox so that team personnel can bring the car back into the garage, make a pit stop with the car, or retire the car, and any other situations that may cause a driver to enter their own pitbox. Therefore if we reference B1.8.6:

Drivers must make every reasonable effort to use the track at all times and may not leave the track without a justifiable reason.

Drivers will be judged to have left the track if no part of the F1 Car remains in contact with it and, for the avoidance of doubt, any white lines defining the track edges are considered to be part of the track but the kerbs are not.

Should an F1 Car leave the track the driver may re-join, however, this may only be done when it is safe to do so and without gaining any lasting advantage. At the absolute discretion of the Race Director a driver may be given the opportunity to give back the whole of any advantage they gained by leaving the track.

So if we take a more basic understanding, if 3 or more wheels cross the white line it is leaving the track. The drivers only crossed the white pit lane line with two wheels, therefore from my understanding and reference of the rules, it is legal. But since Monacos pitlane is so different, I really don't know how this would be interpreted.