More incidents on a day they were cooped up indoors doesn't sound surprising. It sounded when you were saying it's gone from 2 incidents to 7 that this was a general pattern of things increasing over a few weeks, not just one day to the next.
Young children often smile or laugh when they are doing something they know is naughty - it does not mean they are getting some kind of perverse kick out of it. It means they are testing a boundary. This is completely normal. He might need a clearer response signalling that the behaviour is unacceptable - redirection might be inadvertantly rewarding as others have said. They would do better to look for patterns in his behaviour/body language before an incident (this might be easier to observe on CCTV recordings if they have it) so they can redirect before the incident happens, and/or to generally increase supervision when he is near other children indoors, or try to ensure he is constantly occupied so that he doesn't get bored.
Some of the responses on this thread are quite frankly bonkers. It's common for children to go through a hitting/pushing/biting phase, especially when they have just gained a baby sibling, and the reaction doesn't need to be pulling them out of childcare. The parent should take it seriously and try to address it if it occurs at home, as well as looking at any developmental concerns, and stay in communication with the nursery (which OP is doing all of). If the child with challenging behaviour seems to be extremely distressed by the environment, then it might be worth looking at reducing time or removing them from said environment, but it doesn't sound like they are at that point yet.
I completely understand for the parent of the child who was injured that it can be triggering to read about these things, but children of this age do get physical with each other and nurseries need policies to safeguard children from it. Gaining a concussion from such an incident is horrific and must have been awful to experience, however I imagine this is very rare to the point it must have had an element of bad luck. You cannot eliminate all risk at nursery, and toddlers hitting or pushing each other doesn't usually cause concussion.
I don't think that physical play with toddlers like throwing them around, tickling etc counts as "play fighting" and it has lots of benefits apparently, in any case I certainly would not stop a form of play which he enjoys and engages with, as positive supportive relationships with adults who love him are incredibly important for young children. I think some wording by OP is being taken out of context by other posters and exaggerated, although of course this might be my interpretation.
I did look at the ASQ for 27 months and am a bit confused about saying DC meets all the criteria for communication if he will not follow directions, as four of the six questions are related to following simple directions, the other two are using pronouns correctly and putting 3-4 words together, which I am not sure if DC is doing (you said he has started putting words together "sometimes"). But even if he was doing both of those things consistently (not "sometimes") the points for those two questions do not take him out of the dark shaded area, which indicates further investigation is likely to be needed.
The lack of any other collaborative play is extremely unusual - have you done an M-CHAT questionnaire? That might be one to discuss with the health visitor as some of the questions can be a bit ambiguous, but the versions for HCPs have much more specific clarifications with examples.