I assume they were not allowed to eat.
You assume wrong. The regulations for the match were published by FIDE here. There is no mention of food not being allowed. Article 4.10.5 defines what they may and may not bring into the match:
4.10.5. The players are not permitted to bring into the playing area telephone, technical and other equipment extraneous to play, which may in any way disturb or upset the opponent. The Chief Arbiter decides what constitutes extraneous equipment disturbing the opponent.
If the Chief Arbiter allows them to bring food, e.g. sandwiches, a banana, yoghurt then that is allowed.
similar issues would arise if the players could go to the cafeteria in the building etc
The areas where the players are allowed to go during play are called the "playing venue" and the extent is normally defined by the Chief Arbiter. In this case it would seem to be the playing area, the toilets and the "rest lounge area". This is defined in the above FIDE document.
4.10.3. Both players have access to the same toilet facilities during the games. There are no separate rest rooms for the players during the games. Both players use the same rest lounge area which is visible by the Chief Arbiter and spectators.
With the permission of the Chief Arbiter I'm sure they would be allowed to bring their own snacks with them to eat in the "rest lounge area". Eating at the board is normally not allowed as this would likely disturb the opponent.
Any "lunch break", even at the table, would raise clock issues
No. First eating at the table is not normally allowed. Second, if you pay attention during the match you will see that there are natural breaks when the players get more time added on. The first break is at 40 moves when the players get an additional hour on the clocks.
It is common in top level matches that players will take a few minutes break from the game at this first time control to go to the toilet, maybe splash water in their face, grab a drink or sandwich. This chance for refreshment and relief costs a few minutes on the clock but can be important in a long game.
The second time control comes after an additional 20 moves and the extra time is only 15 minutes for the rest of the game. Of course the players get an increment of 30 seconds every time they make a move but the 15 minutes is not a lot of time and it is more unusual for players to take a break here.
Does this prohibition extend to snacks like a chocolate bar?
There is no prohibition.
Is there a rule what can be drunk?
FIDE subscribes to the WADA, the World Anti-Doping Agency, and has adopted their definitions and rules. The FIDE document Chess WADA, Anti-Doping Policy, Nutrition and Health says this:
The 2018 WADA Prohibited List and Monitoring Program can be found at:
The most relevant banned substances for chess are:
• Amphetamines – e.g. Adderall, Ritalin
• Ephedrine and Methylephedrine – Prohibited by WADA when its
concentration in urine is greater than 10 micrograms per milliliter
• Pseudoephedrine is prohibited when its concentration in urine is greater than 150 micrograms per milliliter
Substances not present on the Prohibited List but represented in the Monitoring Program:
• Caffeine – Included in WADA Monitoring Program and relevant for incompetition testing only. Any urine test reading of less than 12 micrograms
per milliliter poses no problem.
• Codeine – A common ingredient in, for example, preparations used to treat
coughs and stomach upsets. Any dosage is highly unlikely to be significant
when taken in normal therapeutic quantities.
As long as these are obeyed there is no problem with drinks.
I suppose the players can go to the toilet during games; what if they got a burrito on the way from a friend and devoured it on the way?
No spectators are allowed in this area. The possibility does not arise. In any case players are provided with refreshments. According to the match regulations:
4.5.4. The Organizer provides, free of charge, coffee, tea, soft drinks and snacks for the players, the principals, VIPs and accredited