If you install the Core Property converters package - https://our.umbraco.org/projects/developer-tools/umbraco-core-property-value-converters/ - then you should be able to do:

.Where(x => x.ContentType.Alias == "Meeting" && x.GetPropertyValue<IEnumerable<IPublishedContent>>("topics")
.Select(p => p.Id).
Contains(Model.Content.Id));

I think this reads a bit better, but would guess it might perform slightly worse.

I often use a static helper method something like:

/// <summary>
/// Converts a delimitted string to an enumarable of integers
/// </summary>
/// <param name="csvString">The CSV string</param>
/// <param name="separator">The delimiter character</param>
/// <param name="addDefaultOnFail">Set to true to add the default value for type T if the conversion fails</param>
/// <returns>A list of integers</returns>
public static IEnumerable<int> ToIntCsvList(string csvString, char separator = ',', bool addDefaultOnFail = false)
{
    if (String.IsNullOrEmpty(csvString))
        yield break;

    var values = csvString.Split(separator);
    int d;

    foreach (var item in values)
    {
        if (int.TryParse(item, out d))
        {
            yield return d;
        }
        else if (addDefaultOnFail)
        {
            yield return default(int);
        }
    }

    yield break;
}

You can then call it like this:

.Where(x => x.ContentType.Alias == "Meeting" &&
StringHelper.ToIntCsvList(x.GetProperty("topics").DataValue.ToString()).Contains(Model.Content.Id));

Again, not sure if it's more readable, but should be efficient and more resilient.