This exactly. And because this is open-source, its super easy to track this down. I searched for the source of the error message:

https://github.com/iputils/iputils/blob/3400f3a740942064a545f02aabcf68e234733297/ping/ping.c#L237

see this is ping_strtod function - so search for that function name and the first hit is the '-i' case:

		case 'i':
		{
			double optval;

			optval = ping_strtod(optarg, _("bad timing interval"));
			if (isless(optval, 0) || isgreater(optval, (double)INT_MAX / 1000))
				error(2, 0, _("bad timing interval: %s"), optarg);
			rts.interval = (int)(optval * 1000);
			rts.opt_interval = 1;
		}
			break;