Yes, content is absolutely the number one factor for SEO.
Furthermore, some of the common recommended SEO strategies have the potential to actually penalise your site in the Google index and SERP ranking. Backlink trading and leaving backlinks on poor quality sites are both signals that will actually de-rank your website, as well injecting keywords that bear no relevance to the actual website content. Certain SEO strategies, like so-called SEO-friendly URLs I believe are completely useless. Google knows how to find the topic heading in phpBB posts and knows how to parse phpBB URLs.
That said, I disagree that all SEO strategies are useless. I do think there is scope to improve SEO on phpBB.
One potential improvement is to remove redundant f=# query flags in viewtopic.php URLs. Google thinks viewtopic.php?t=# is canonical but when it crawls via internal phpBB links, it sees the structure of URLs as viewtopic.php?f=#&t=#. Google does identify these links as non-canonical duplicates but I do not think they would have the same strength as backlinks as if the canonical URL viewtopic.php?t=# was the default. Also, the sheer number of these non-canonical duplicates might be a signal to Google that the site is poor quality or full of rubbish URSL and to then de-index good quality URLs. I see there is a recent pull request on GitHub to fix this issue. Amongst the good reasons to implement this change, I think it may also help SEO.
Another factor which I think is downplayed are sitemaps. I have never had a sitemap and Google had most of my board indexed. I therefore assumed like many on this board have commented that sitemaps are useless. However, in the past 6 months, a large number of pages have gradually been de-indexed. There are no crawl errors etc. When I submitted a sitemap, the number of URLs put into the sitemap immediately started jumping up again, as did the impression rate. I only have 2 weeks of data on this, the results so far are encouraging (10% more URLs in the index). It also makes the data in Google Search Console much easier to track and analyse. I intend to report back when I have more definitive data to go on.
Finally, I also suspect that while it does not directly improve SERP ranking and the number of raw impressions, SEO metadata using OpenGraph and JSON-LD etc can help click-through rates because they make links to the site more attractive and informative. This in turn would indirectly improve the SERP rank over time.